The frequency of sexual imagery outside of sexual behavior was assessed. In addition, an attempt was made to determine the viability of a distinction between internally generated sexual thoughts (fantasy) and externally provoked sexual thoughts (urge). After a 3-day practice period, 49 male and 47 female heterosexual undergraduates self-monitored the frequency of fantasies, urges, and masturbatory fantasies for 7 consecutive days. The most significant finding was that men reported a significantly greater frequency of sexual urges and masturbatory fantasies than women. No gender differences emerged with respect to fantasies. These findings suggest that a distinction between internally and externally generated sexual imagery is viable and may be useful in future studies examining the frequency, functions, and determinants of sexual imagery.