BACKGROUND The aim of this study was to know the prevalence, epidemiology, clinical manifestations and analytical changes present in anti-HCV positive blood donors detected in Asturias. METHODS A prospective analysis of the incidence and prevalence of anti-HCV positivity in the blood donations carried out in Asturias from October 1989 to October 1991 was performed, as was a clinical and analytical study of the anti-HCV positive cases who attended a clinic specifically created for the same. RESULTS The prevalence of the anti-HCV was 0.87% of the donors (372/42,789) and 0.50% of the donations (372/73,831) being higher among new donors (1.77%, 165/9,322). Of the 288 cases studied (77.4%), only 51 (17.7%) had been transfused and 105 (36.5%) lacked the previous parenteral risk factor. Only 31 (10.8%) presented symptoms or signs of liver disease and the positivity of the anti-HBc was not associated to any relevant analytical change. The existence of previous major surgery or transfusion was variable with the independent predictive value versus a negative anti-HCV control group. The mean follow up was 12.4 +/- 7.3 months (6-30 months) with an increase in aminotransferases (ALT) being detected in 108 cases (52.7%). A good correlation was found between this data, an ELISA-2 score greater than 5 and RIBA-2 positivity: of the 177 cases in whom RIBA-2 was determined this was found to be positive in 109 (61.6%); 84 cases (77.1%) had an increase in ALT and 100 (91.8%) an ELISA-2 score greater than 5. CONCLUSIONS In Asturias the prevalence of anti-HCV positivity among blood donors is almost 1% and is greater if new donors are considered, being confirmed by RIBA-2 in 61% of the cases. The subjects are usually asymptomatic and up to one third of the same lack any known risk factor, while almost half have hypertransaminasemia during follow up.