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The main conclusion of the multicentre study on factors determining the differential spread of HIV in four African cities was that differences in sexual behaviour could not, by themselves, explain the differences in HIV prevalence between the four cities. The present paper examines three potential sources of bias that could invalidate this conclusion: (1) changes in sexual behaviour since the start of the HIV epidemics; (2) bias due to the low response rates of men; and (3) bias in reported sexual behaviour.To assess whether there have been any changes in sexual behaviour over time, selected parameters of sexual behaviour were compared between different age groups in the four cities. The maximum likely extent of bias due to non-participation of men in Yaoundé, Kisumu and Ndola was assessed with a simulation exercise, in which records of non-participants were replaced with records of 'low activity men' in Yaoundé and 'high activity men' in Kisumu and Ndola. To assess the validity of the sexual behaviour data, internal validity checks were carried out: comparing biological data on sexually transmitted infections with reports; comparing reports of spouses; and comparing numbers of sex partners reported by men and women. A fourth method consisted of comparing the findings of the multicentre study with an external source, Demographic and Health Surveys (DHS).There were differences in sexual behaviour between the younger and the older age groups in all four cities but there was no evidence of a shift towards safer sexual behaviour in the high HIV prevalence cities. After simulating results for male non-participants in Yaoundé, Kisumu and Ndola, the median lifetime number of sex partners was similar in Yaoundé, Kisumu and Ndola. By testing for various sexually transmitted infections among men and women aged 15-24 years who reported that they had never had sexual intercourse, we could establish that, in all four cities, at least 1-9% of men and 6-18% of women had misreported their sexual activity. The number of non-spousal partners in the past 12 months reported by men was two to three times higher than the number reported by women, as has been found in other studies. The most consistent differences between our survey and the DHS were found in the numbers of non-spousal partners in the past 12 months reported by never-married men and women. In all four cities, participants reported more non-spousal partners in the DHS than in our survey.In all four cities, we found evidence that men as well as women misreported their sexual behaviour, but overall it seems that under-reporting of sexual activity was not more common or more serious in the two high HIV prevalence cities than in the two low HIV prevalence cities. We believe that the main conclusions of the multicentre study still hold.