The aim of our study was to determine prevalence of hepatitis C virus infection (HCV) among patients with haematological and oncological diseases treated in the past with blood or blood products (about 20% among adults and about 50% in children in selected wards in 1992). Infection with HCV was detected in 3 out of 64 examined patients (4.7%) and in 64 of 14 098 blood donors (0.4%), p < 0.05. The two HCV cases in the study group were in persons with haemophilia A treated with cryoprecipitate before 1992. Among 51 persons receiving transfusions only after 1992--1 case of HCV infection was detected (2.0%) Our results indicate improvement in the safety of treatment with blood and/or blood products and suggest that the present risk of HCV infection among patients with haematological and oncological diseases treated with these products after 1992 is rather small.