Condoms are most effective when they are applied before intercourse. This study assessed the prevalence of 'condom cheats'--instances of condom use in which the condom is put on after initial penetration. As part of a prospective study of safer sexual behaviour, 103 heterosexual tertiary students completed a condom use diary over a period of up to 6 months. Of the 464 condoms used by study participants, 13% were put on after initial penetration. Thirty-eight percent of condom users reported at least one instance of late application of a condom. Late application of condoms is common and places individuals at risk of infection with HIV and many other STDs. The widespread practice of this behaviour casts doubt on the validity of self-reports of condom use. Assessments of risk based on self-reported condom use may lead to underestimates of the risk entailed in heterosexual young adults' condom use behaviour, and may also lead to underestimates of condom efficacy.