The number of inclusion-forming units (IFUs) observed in quantitative chlamydial cultures may be a surrogate for infectivity or transmissibility. Therefore, we conducted a cross-sectional study of 11,034 patients with Chlamydia trachomatis infection who presented to the Seattle-King County public health department clinics between 1988 and 1996, to determine relationships between the number of IFUs observed in culture and sex, age, race, and serovar class. Of the 11,034 cases of infection we studied, 6801 (62%) were cervical infections in women, and 4233 (38%) were urethral infections in men. The median count was 450 IFU for women and 72 IFU for men (P<.001). Overall, both men and women infected with B-class serovars had significantly higher IFU counts than did those infected with C-class serovars (P<.001). The median IFU count fell consistently with increasing age for both women (625 IFU for those <16 years old to 185 IFU for those >30 years old; P<.001) and men (210 IFU for those <16 years old to 40.5 IFU for those >30 years old; P<.001). We found, by use of multiple regression analysis, that sex, age, race, and serovar class remained independently related to IFU count, with counts being highest among young white women infected with B-class serovars.