Over a 12-year period (1979-1991), 29 cases of Sweet's syndrome (SS) were detected in the area of Geneva; 24 patients (82%) could be contacted for follow-up. The majority of SS occurred between 1979 and 1986, then from 1991 on. The annual distribution and especially the absence of cases during 4 years suggest an environmental or even infectious factor in SS. Only 4 cases had an underlying disease (ulcerative colitis, polycythemia, lymphoma, sarcoidosis) whereas in the other, a long-term follow-up, including iterative hematological evaluation, did not reveal any underlying condition. Twelve patients were treated by colchicine, 9 by potassium iodide and 5 by prednisone; in 3 patients remission was spontaneous. Nine patients (28%) relapsed; there was no relationship between the type of drugs used and the frequency of relapses.