About a quarter of familial Mediterranean fever (FMF) patients have recurrent painful attacks of polyserositis despite regular colchicine treatment. There is no known alternative drug for colchicine-resistant cases. We had previously observed a patient with FMF whose painful attacks disappeared during the 6 month period of interferon alpha (IFN) treatment for his chronic hepatitis B. The objective of the present study was to investigate the possible beneficial effect of IFN on these episodes. Twenty-one consecutive attacks in seven adult patients with FMF were treated at early onset with IFN, the dosage being 3-10 million I U s.c. Eighteen of the 21 attacks could be halted in a mean time of 3.05 h, while the intensity of abdominal pain remained very low. Observed side-effects were generally mild and acceptable. IFN may be a useful adjunct for the treatment of colchicine-resistant attacks in FMF patients.