OBJECTIVES: To estimate HIV prevalence in various blood donor populations, to identity sociodemographic risk factors associated with prevalent HIV and to assess the feasibility of offering routine voluntary counselling services to blood donors. DESIGN: Cross-sectional study. SETTING: Thyolo district, Malawi. METHODS: Data analysis involving blood donors who underwent voluntary counselling and HIV testing between January 1998 and July 2000. RESULTS: Crude HIV prevalence was 22%, while the age standardised prevalence (>15 years) was 17%. Prevalence was lowest among rural donors, students and in males of the age group 15-19 years. There was a highly significant positive association of HIV prevalence with increasing urbanisation. Significant risk factors associated with prevalence for both male and female donors included having a business-related occupation, living in a semi-urban or urban area and being in the age group 25-29 years for females and 30-34 years for males. All blood donors were pre-test counselled and 90% were post test counselled in 2000. CONCLUSIONS: HIV prevalence in blood donors was alarmingly high, raising important concerns on the potential dangers of HIV transmission through blood transfusions. Limiting blood transfusions, use of a highly sensitive screening test, and pre-donation selection of donors is important. The experience also shows that it is feasible to offer pre and post test counselling services for blood donors as an entry point for early diagnosis of asymptomatic HIV infection and, broader preventive strategies including the potential of early access to drugs, for the prevention of opportunistic infections.