To monitor the impact of its AIDS prevention efforts, Switzerland conducts an annual telephone survey about AIDS-related knowledge, attitudes, and preventive behavior in the general population aged 17-45 years. Based upon a test-retest study with additional focused interviews conducted upon a subsample of 138 respondents aged 17-22, the authors report their findings from an investigation of the validity and reliability of indicators of sexual behavior and condom use in those surveys. The initial survey sample was comprised of 2800 respondents aged 17-45 in October 1992. The subsample included more French-speaking respondents and more people in stable relationships than did the initial sample. Survey indicators are the number of lifetime sex partners, casual sex during lifetime and number of casual sex partners during the preceding 6 months, condom use with casual partners during the past 6 months, the acquisition of a new steady partner during the year, condom use with that partner, age at first intercourse, and condom use at last intercourse. These indicators were determined to be highly reliable, although more research is needed on the validity of the data. Among sexually active respondents, 12.5% of the men and 1.9% of women included nonpenetrative sex in their definitions of sexual intercourse. Furthermore, 40% of the 15 respondents who reported consistent condom use with casual sex partners at the initial interview later admitted at reinterview that they sometimes have unprotected sex.