A fossilised Neanderthal rib found in a shallow cave at Krapina, Croatia, shows signs of a bone tumour.
The discovery is the oldest evidence yet of a tumour in the human fossil record, say US scientists.
The research, published in the journal PLOS One, gives clues to the complex history of cancer in humans.
Until now, the earliest known bone cancers have been identified in ancient Egyptian remains from about 1,000-4,000 years ago.
"It's the oldest tumour found in the human fossil record," Dr David Frayer, the University of Kansas anthropologist who led the US team, told BBC News.
"It shows that living in a relatively unpolluted environment doesn't necessarily protect you against cancer, even if you were a Neanderthal living 120,000 years ago."