ObjectiveTo measure types of sex role prevalence in common and risk-related behaviours among gay men for modelling HIV transmission. DesignCohort study of 385 homosexually active men recording sexual diaries over 1-month periods. MethodsMeasures of incidence of behavioural sex roles for masturbation, fellatio, anal intercourse and anilingus by relationship type, derived from 1-month sexual diary data. ResultsLow behavioural role rigidity for masturbation and fellatio, but higher rigidity for anal intercourse and anilingus. Participants with no regular partner showed a relatively low frequency of anal intercourse, whereas those in closed relationships showed a high frequency. ConclusionAlthough anal intercourse shows a certain degree of behavioural role rigidity, this rigidity is not large enough to conclude that gay men exclusively engage in either an active or a passive role. Typical rates for exclusive active and passive roles for anal intercourse during the month the diaries were recorded were in the range of 12–15%; the dual role was significantly higher.