We compared findings on sexual behavior from a self-administered questionnaire and a face-to-face interview, separated by 2–4 weeks, directed at 383 Italian secondary school students. The correlation coefficients or kappa values for reported prevalence of coital experience, age at first inter-course, and age of first partner were all greater than 0.7. The interview underreported coitus and overreported condom use, when administered before the questionnaire. The students thought that 18.4% of their classmates had ever had coitus, compared with 23.5% on self-report. We conclude that despite an overall high reproducibility of sexual behavior measures, distortions due to perceptions of social desirability might occur. (Epidemiology 1992;3:531–535)