Twelve cases of leptospirosis followed by the Infectious Diseases Clinic of the Cukurova University Medical School, Adana, Turkey, between January 1994 and November 1995 are reported. Included are their clinical manifestation, laboratory findings and serotype. Nine men and three women with an average age of 40.4 years were studied. Symptoms, clinical manifestations, laboratory findings and treatment of the disease are evaluated. All of the patients had fever and chills and the following symptoms: nausea and vomiting (91.6%), lower back pain and myalgia (58.3%), headache (50%), epistaxis (16.6%) and confusion (25%). The predominant clinical findings were jaundice (91.6%), hepatomegaly (41.6%), dyspnea (25%), conjunctival suffusion (33%), and nuchal rigidity (33%). Diagnosis was based on dark-field examination of the blood, cerebrospinal fluid and urine specimens. Also, microscopic agglutination tests (MAT) were carried out for serodiagnosis. MAT showed L. interrogans serovar icterohaemorrhagiae in 11 cases and L. interrogans serovar grippomosocova in one case. There was cross reaction with L. biflexa patoc in all cases. Agglutinations were tested in the same specimens twice and confirmed in the Microbiology Laboratory of the Etlik Veterinary Research Institute in Ankara. All cases were treated with penicillin and doxycycline. In the end; 83.3% of the patients were cured and 16.6% died due to hepatorenal failure.