Background Many aspects of sexual behaviour are relevant for the risk of acquiring a sexually transmitted disease (STD). Both the frequency of intercourse and the frequency of partner change are likely to play a role, but their distributions are not independent. This work aims to evaluate their joint contribution to the risk of chlamydia and HIV using a new concept, the individual risk index.Methods The basic reproductive ratio, R0, for an STD in a population can be defined from average intercourse and partner frequencies in the population, and from the duration and infectiousness of the disease. We calculate an individual risk index by entering a persons intercourse and partner frequencies into the R0 formula, and define as potentially riskful those behaviours that lead to index values greater than one for a given infection. The index was analysed on 10915 subjects aged 18 to 52 years from two surveys on sexual behaviour conducted in 1987 and 1992, using HIV and chlamydial infection as examples.Results The distribution of the index depends on the joint distribution of intercourse and partner frequency. The potential risk groups for HIV and chlamydia overlapped only partly, 1.9% were potential risk takers for both infections, 0.9% for HIV only and 3.3% for chlamydial infection only. Sexual behaviour differed strikingly between the groups. For the HIV only group, the mean values for intercourses and new partners per year were 200 and 0.6, respectively. For the chlamydia only group these values were 37 and 2.4 per year. Potential risk taking was 15 times higher among singles compared to married/cohabiting subjects, and decreased from 1987 to -92 for both infections.Conclusions A new STD risk index may be defined based on the R0 concept. The index has a meaningful interpretation, and can be used to characterise risk groups and compare populations.

*Chlamydia Infections/TRANSMISSION *Sexually Transmitted Diseases/TRANSMISSION

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