In November 2015, the biography of Frederick Trump, Donald Trump's grandfather, was condensed into two paragraphs and then passed around the internet via a meme. While some of the information included in the meme is accurate, much of it is either over-exaggerated or incomplete:
This particular rumor centers on the idea that Frederich Trump made his fortune via brothels and opium dens. While there is anecdotal evidence that Trump dabbled in prostitution, there is no proof that this made up the bulk of his fortune.
In Gwenda Blair's 2000 book The Trumps: Three Generations of Builders and a Presidential Candidate, she described how Frederick Trump opened a series of restaurants and hotels during the Klondike Gold Rush in the 1890s. One of those hotels, The Arctic Restaurant and Hotel, was described as decadent and far superior to other restaurants in the area:
“In the larder was salmon and an extraordinary variety of meats, including duck, ptarmigan, grouse, goose, and swan, as well as caribou, moose, goat, sheep, rabbit, and squirrel. Incredibly, the New Arctic served fresh fruit: red currants, raspberries, strawberries, blueberries, blackberries, even cranberries. A small oasis of luxury, the Arctic’s menu was a vast improvement over what the two restaurateurs had been able to offer on the trail.”
An anonymous letter to the Yukon, however, claimed that The Arctic Restaurant and Hotel was also known for prostitution:
“I would advise respectable women traveling alone, or with an escort, to be careful in their selection of hotels at Bennett,” he wrote. “For single men the Arctic has excellent accommodations as well as the best restaurant in Bennett, but I would not advise respectable women to go there to sleep as they are liable to hear that which would be repugnant to their feelings – and uttered, too, by the depraved of their own sex.”
While it's unclear if Frederich Trump directly profited from prostitution at his hotel (or whether it even occurred there), it should be noted that the world's oldest profession was relatively commonplace during the Gold Rush.
The meme also claims that Frederick Trump decided to go back to Germany when police started cracking down on "his criminal rackets." Again, this is based on little more than a morsel of truth, and does not tell the whole story. In 1901, Trump sold his assets and returned to Germany. While one could argue that Trump made the decision because he believed that police were going to start enforcing prostitution laws, that is only one factor that led to Trump's departure for Germany:
Frederick Trump saw that it was time to leave. If Major Wood actually enforced the laws regarding prostitution, gambling, and liquor, hotels and restaurants would be far less profitable. Not only that, the economic boom was bound to be short lived. There was not nearly enough solid economic development to absorb these newcomers in any long-term way; when the placer deposits were emptied, they would go back home. Without the umbrella of gold, other local industries would not be strong enough to keep going on their own and compete with cheaper sources farther south. The boom was over, Frederick Trump realized...
... Frederick Trump left just in time. He avoided the uproar when his erstwhile partner hit the skids, and he avoided the economic decline that would soon sweep over White Horse. Once again, in a situation that created many losers, he managed to emerge a winner. He had made money; perhaps even more unusual in the Yukon, he had also kept it and departed from White Horse with a substantial nest egg. He had accomplished this goal of making and saving enough money to marry. But he had no intention of doing so in America. For this important moment, he would have to return to Germany.
While the meme exaggerated Trump's involvement in "criminal rackets," it did correctly state that Trump returned to the United States after the German government determined that he had originally left Germany in 1885 to avoid taxes and the army.
In summation, Donald Trump's grandfather Frederick Trump was a German immigrant who made his fortune by opening several restaurants and hotels in Seattle and British Columbia during the Yukon Gold Rush. While some of these hotels may have been used for prostitution, gambling, or other seedy activities common on the trail, it is incorrect to say that Trump built his fortune on illegal activities.