A 31-year-old man with Crohn's disease developed arthritis, spiking fever, and skin rash indistinguishable from that of adult-onset Still's disease. He was admitted to our hospital because of a periumbilical intestinal skin fistula. Crohn's disease had been diagnosed in 1991, and had required intestinal resection twice, and schizophrenia had been diagnosed in 1993. He developed polyarthritis and spiking fever, accompanied by a macular skin rash on both forearms. Marked hepatosplenomegaly and bilateral pleural effusion were detected on computed tomography examination. These findings are indistinguishable from those of adult-onset Still's disease. Because his mental status had deteriorated following high-dose prednisolone on a previous admission, he was treated with an immunosuppressive agent on this occasion, with the treatment being successful. This is the first report of adult-onset Still's disease complicating Crohn's disease. In patients with Crohn's disease, polyarthritis and skin rash can easily be misdiagnosed as enteropathic arthritis with erythema nodosum associated with the Crohn's disease. Although adult-onset Still's disease may not be fatal, early diagnosis is important because it can, in rare cases, result in life-threatening complications.