Chlamydia trachomitis is the most common sexually transmitted infection in the UK and the number of cases diagnosed each year continues to rise. Although much is known about the risk factors for chlamydia from previous observational studies, less is known about how individuals put themselves at risk. Do they engage in just one risky type of behaviour or are certain individuals 'risky', engaging in multiple risky behaviours? This paper uses latent class analysis, applied to the National Survey of Sexual Attitudes and Lifestyles II (2000-2001), to determine whether a subgroup of high-risk individuals can be identified and explores which features of their behaviour distinguish them from other groups of lower risk individuals. A 3-class solution was obtained, splitting the sample on the basis of the number of sexual partners in the past year. Those with no sexual partners in the past year (8%) and one sexual partner in the past year (71%) were much less likely to have engaged in any of the other behaviours known to increase chlamydia risk. However, the group who had two or more sexual partners in the past year (21%) were much more likely to have also engaged in other risky behaviours. The number of partners in the past year is therefore a useful marker for identifying those at increased risk of chlamydia infection. Individuals under 25 years old, males and those who were single or previously married were more likely to be allocated to the risky group. However, in spite of observed higher incidence of chlamydia infection, individuals in the black ethnic minority groups did not show an increased prevalence of risky behaviour, after controlling for age, sex and marital status.