Seropositivities to human papillomavirus types 16, 18, or 33 capsids and to Chlamydia trachomatis are markers of sexual behavior.

The association of seropositivity to human papillomavirus (HPV) capsids of types 11, 16, 18, or 33 with sexual behavior was investigated. Among 1002 women visiting family planning or youth clinics in Sweden, an age-matched subsample of 274 women stratified according to lifetime number of sex partners was analyzed. The proportion of HPV-16-seropositive subjects increased linearly at approximately 4% per partner (P < .001), from 4% among those with 1 lifetime partner to 35% among those with >5 lifetime partners. Also, HPV-33 and HPV-18 seroprevalences were linearly dependent on the number of partners (P < .001, increase with 4% per partner, and P = .008, increase with approximately 3% per partner, respectively), providing serologic confirmation that the important mode of transmission of HPV-16, -18, or -33 infection in women is sexual. HPV serology appears to be suitable as a marker of sexual behavior in populations.