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 intercourse, 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.