Hitler probably didn't have any Jewish blood, it can't be completely ruled out. Hitler's father was illegitimate and to this day there is some question about who his grandfather was. Throughout his career he was dogged by rumors about his pedigree, some of them circulated by his fellow Nazis. In 1933 the London Daily Mirror published a picture of a gravestone in a Jewish cemetery in Bucharest inscribed with some Hebrew characters and the name Adolf Hitler. It's now known the Bucharest Hitler could not have been grandfather to our Adolf, but Hitler was sufficiently worried about the whole business that, according to the historian John Toland, he had the Nazi law defining Jewishness written to exclude Jesus Christ and himself.
Here's what we know: Hitler's paternal grandmother, Maria Schicklgruber, gave birth to Alois, Hitler's father, in 1837. She was 42 and unmarried at the time and apparently never revealed the father's identity. Five years later she married Johann Georg Hiedler or Hitler (spelling was a bit casual in those days). But Alois kept the surname Schicklgruber until he was 39 years old.
In 1876 a new baptismal certificate was issued declaring that Alois's stepfather J.G. Hiedler was in fact his real father. By this time both Maria and J.G. were dead. Why the name change so late in the day nobody really knows, but there is speculation that Alois did it so he could come into an inheritance. At any rate, few researchers today believe J.G. was really Alois's father.
Now for the weird stuff. After the war Hitler's former lawyer, Hans Frank, claimed that Adolf told him in 1930 that one of his relatives was trying to blackmail him by threatening to reveal his alleged Jewish ancestry. Hitler asked Frank to find out the facts. Frank says he determined that at the time Maria Schicklgruber gave birth to Alois, she was working as a household cook in the town of Graz. Her employers were a Jewish family named Frankenberger, who had a 19-year-old son. The son, according to Frank, was Alois's father and Hitler's grandfather--which would make the man who inspired the Holocaust one-quarter Jewish.
Frank's allegations have vexed historians ever since. The distinguished Hitler scholar Werner Maser was so irritated he claimed Frank made the whole thing up. Others think Frank was telling the truth but that the research he did for Hitler was faulty. It turns out that all Jews had been expelled from Graz in the 15th century and were not allowed to return until the 1860s; what's more, so far as can be determined, Maria Schicklgruber never lived in Graz. Frank's source for the Frankenberger yarn was a distant relation of Hitler's, who supposedly had letters exchanged by the Frankenbergers and Maria Schicklgruber. (It's claimed they gave her child support.) But neither the relative nor the letters have ever surfaced, and chances are it's all a crock.
So who really was Hitler's grandfather? Werner Maser thinks it was the brother of his legal grandfather, one Johann Nepomuk Hiedler. But that's not all. J. Nepomuk was also the grandfather of Klara Poelzl, Hitler's mom. In other words, J.N. was both Adolph's paternal grandfather and his maternal great-grandfather. I'm not about to tell you any of this was the proximate cause of Hitler's persecution of the Jews, his suicide, or anything else. Still, if you believe Maser, not only was Hitler twisted, so was his family tree