Early diagnosis of leptospirosis is important because severe leptospiral infection can run a fulminant course. The polymerase chain reaction (PCR) was evaluated for the detection of leptospires in clinical samples from patients with acute leptospiral infection. Blood and urine samples from 71 patients with leptospirosis were examined by PCR, culture or serology. Samples from 44 (62%) patients with the diagnosis of leptospirosis were positive by PCR as compared to 34 (48%) by culture. The presence of leptospires was demonstrated by PCR in 13 patients before the development of antibodies, as well as in two patients who were seronegative during their illness and at autopsy. Samples from 16 patients without leptospirosis were seronegative and culture negative, and also negative by PCR. We conclude that PCR is a rapid, sensitive and specific means of diagnosing leptospiral infection, especially during the first few days of the disease.