Retrospectively have been studied clinical signs and routine laboratory investigations of patients with leptospirosis (n(1)=94) and acute viral hepatitis (AVH) (n(2)=1705). The comparison of results in two groups have revealed significant differences in the frequency of acute onset in leptospirosis and AVH with fever (respectively in 100% and 58,01%; p<0,005); pains in calf muscles (in leptospirosis in 85,11%, in AVH have not been observed); headache (resp. in 68,08 and 18,96%; p<0,005); anorexia (resp. in 64,89 and 87,36%; p<0,005); lumbar pains (in leptospirosis in 40,43%, in AVH have not been observed); clay colored stool(resp. in 6,38 and 93,60%; p<0,005); heaviness in abdomen(resp. in 3,19 and 84,38%; p<0,005); hepatic tenderness (resp. in 41,49 and 76,63%; p<0,005); conjunctival injection only in leptospirosis (86,17%). Routine laboratory investigations have been demonstrated anemia, leucocytosis, granulocytosis, increased erythrocytes sedimentation rate(ESR) and thrombocytopenia significantly often in leptospirosis than in AVH (p<0,005). The comparison of results of liver biochemical investigations have been established increased serum bilirubin in leptospirosis and AVH(resp. in 75,00 and 93,60%; p<0,05); mildly increased aminotransferases activities in leptospirosis and extremely increased in AVH (p<0,005); increased creatinkynase in leptospirosis; increased fibrinogen level in leptospirosis (av. 6,74 g/L), normal to decreased in AVH (av. 3,01 g/L) ( p<0,005). Nitrogen parameters have been increased in leptospirosis with ARF.