The pathogenesis of thrombocytopenia in chronic hepatitis is not well known. This study evaluated the relationship between liver injury, serum thrombopoietin, splenomegaly and thrombocytopenia in chronic viral hepatitis. Two hundred and nine patients were enrolled, 85 with splenomegaly and 124 without. Thrombocytopenia was present in 71% and 23% of patients with or without splenomegaly respectively. In subjects with low platelet count, those with splenomegaly showed significantly lower platelet numbers than those without splenomegaly. The spleen size correlated with portal hypertension. An inverse correlation between spleen size and platelet count was observed (r = -0.54; P < 0.0001). In patients without splenomegaly, thrombocytopenia was associated with the grade of fibrosis; platelet counts were the highest in patients with fibrosis 0-2, lower in those with grade 3 (P < 0.008) and lowest in those with grade 4 (P < 0.05). These findings were independent of demographic and biochemical characteristics, hepatic necroinflammatory activity, portal hypertension and splenomegaly. Patients with normal platelet counts showed higher thrombopoietin levels than those with low platelet counts (P < 0.0001). An inverse correlation between thrombopoietin levels and fibrosis grade was observed (r = - 0.50; P < 0.0001). Median thrombopoietin levels were 58 and 27 pg/ml for fibrosis grade 0-1 and grade 4 respectively (P < 0.001). These data indicate that advanced hepatic fibrosis, causing an altered production of thrombopoietin and portal hypertension, plays the central role in the pathogenesis of thrombocytopenia in chronic viral hepatitis.