From October 1982 to June 1985 158 hospitalized patients in the National Hospital of Niamey, Republic of Niger, were selected whenever one of the following signs was found: hepatomegaly, jaundice, ascites, oesophageal varices, abdominal venous pattern, or splenomegaly. Investigations included hepatic echography (158/158), needle liver biopsy (68/158), radioimmunoassays for serum hepatitis B surface antigen (HBsAg; 158/158), anti-HBs (152/158), anti-HBc (129/158) and anti-delta antibody (anti-HD; 158/158). 112 patients with liver diseases comprised 28 with chronic hepatitis, 55 with non-alcoholic hepatic cirrhosis, and 29 with hepatocellular carcinoma (HCC). 46 patients with other diagnoses were used as controls. 71/112 liver disease patients were positive for HBsAg in serum compared with 1/46 controls (P less than 10(-9)). Prevalences of delta superinfection in patients with serum HBsAg (+) and anti-HD (+) were 45/112 (40.2%) in liver disease patients versus 1/46 (2.2%) in controls (P less than 10(-9)). Delta superinfection was very frequent in chronic hepatitis (8/28), non-alcoholic cirrhosis (24/55) and HCC (14/29). In chronic hepatitis, delta superinfection was more frequent in the chronic active form than in the chronic persistent type (not significant). Cirrhosis patients with delta superinfection were younger (10 years in males, 11 years in females) than those without (P less than 0.05).