Measurements were made of serum and urine myoglobin in 48 patients with leptospiral jaundice (LJ) and 56 patients with various acute infections. At the height of LJ blood myoglobin level reached 25.96+/-4.3 mu g/l (normal concentration 0.315+/-0.002 mu g/l). Compared to acute pneumonia, acute viral hepatitis, tonsillitis, erysipelas, diphtheria, health values, the ratio of serum myoglobin to urine myoglobin in leptospirosis made up 45.25 against 5.4, 4.8, 6.8, 3.7, 1.8 and 1.3, respectively. A relationship was found between concentrations of myoglobin, bilirubin, creatinine in the blood and leptospirosis severity. Elevation of serum myoglobin as a manifestation of specific myositis is pathognomic for leptospirosis and contributes to the onset of acute renal failure and disturbance of bilirubin metabolism. Quantitation of blood myoglobin may be helpful as an additional test for leptospirosis severity.