['Seropositivity for several sexually transmitted infections (STIs) is often used as a surrogate measure of sexual behavior. The authors assessed the concomitant seropositivity for STIs in women.', 'To estimate the excess of concomitant seropositivity for four STIs among fertile-aged women assuming no coinfections above what would be expected at random.', 'Antibodies to herpes simplex virus type 2, human papillomavirus type 16, HIV, Chlamydia trachomatis, and Treponema pallidum were determined from a random sample of 1110 pregnant women in Tallinn, Estonia.', 'A total of 310 combinations of the concomitant seropositivity were observed, whereas only 193 were expected by chance. Among persons seropositive for two STIs, 78 extra combinations were observed, whereas for three STIs, 35 extra combinations were observed. For four STIs, 3.8 extra combinations were found.'] Is seropositivity to multiple sexually transmitted infections common?