Though only 4.2% of the Danish population live in Ribe County, 12 cases of leptospirosis (22% of the Danish total) were found in the county between 1980-1991. Nine cases were associated with fresh water angling or fish farming. The patients were typically young or middle-aged formerly healthy men, who suddenly became ill with acute febrile disease. Eleven patients survived, one died. Cases were most often seen in late summer and autumn. Four patients required dialysis, and these patients also had relatively high serum levels of bilirubin. Case history and simple laboratory findings are helpful in leading to the diagnosis. Three patients were admitted with the correct diagnosis, and by the third day in hospital all 12 were being treated with relevant antibiotics, eight on suspicion of leptospirosis. Three patients had positive blood cultures on day 4, 6 and 9, and nine patients had late positive serology, two of whom also belonged to the group with positive blood cultures. In two patients the diagnosis was based on clinical findings alone. For these reasons, penicillin should be commenced on clinical suspicion alone.