Between April 1988 and May 1989, 400 males and 400 females attending a Baltimore, Maryland, sexually transmitted disease clinic were enrolled in a study evaluating and comparing behaviors associated with culture-proven gonococcal or chlamydial infection. The subjects were enrolled consecutively, and were all seen by the same clinician. Among participants of each sex, gonorrhea but not chlamydia was associated with increasing numbers of recent (the past 30 days) sexual partners. Compared with males with neither infection, factors independently associated with increased risk of gonorrhea included age less than 20 years (odds ratio (OR) =1.93), the presence of genitourinary symptoms (OR = 8.07), and recent exposure to a new sexual partner (OR =2.78); risk for chlamydial infection in males was associated with genitourinary symptoms (OR = 2.83) and was significantly reduced in those reporting multiple recent (OR = 0.19) or new (OR = 0.07) sexual partners. Among females, age less than 20 years was independently associated with gonococcal (OR = 1.86) and chlamydial (OR =7.79) infections in comparison with females with neither infection. No other behavioral factors were associated with chlamydial infection for females in this study; however, having a regular sexual partner was associated with significantly elevated risk of gonorrhea (OR = 3.85), while the presence of genital tract symptoms was associated with diminished risk (OR = 0.29) for gonorrhea. These data suggest that there are differences in the behaviors associated with gonorrheal and chlamydial infections and that different strategies may be useful in efforts to control these infections. Am J Epidemiol 1992; 136: 662-72 © 1992 by The Johns Hopkins University School of Hygiene and Public Health.