Despite extensive HIV prevention programmes and knowledge of people dying of AIDS, people in Zimbabwe continue to be infected with HIV and other sexually transmitted infections (STIs). This paper presents selected case histories from interviews with 57 men who became HIV positive during follow up of 1678 seronegative male factory workers in Harare, and describes the circumstances in which they were exposed to infection. Youth was a major risk factor, with 47% of those who seroconverted aged between 18 and 24 yr. STIs were reported by 23% of the group in the seroconversion period, a marker of unprotected sex. Individuals did not recognise themselves or their partners as candidates for infection because of categorisation of high risk groups as "promiscuous" or clients of sex workers. Many were optimistic that they had changed sufficiently by using condoms more often or by avoiding sex workers. They made inaccurate assessments of who was safe for unprotected sex, based on judgements about their character, background and age. Over 40% of the seroconvertors had previously been counselled on staying HIV negative. Community approaches which nurture development of supportive group norms, respect for human rights and responsibilities, and safe environments for disclosure of HIV status, are vital for overcoming denial of risk at individual and societal levels. Special efforts targeted at youth are crucial since they have the highest risk of new infections and include use of media, drama, role models, advisory centres, peer education programmes. Health professionals need training and skills to enable people at risk of HIV infection to devise strategies based on more realistic personal risk assessment