Among HIV-negative men who have sex with men (MSM), any incident of unprotected anal intercourse (UAI) between casual partners is usually regarded as risky for HIV transmission. However, men are increasingly using knowledge of their casual partner's HIV-status to reduce HIV risk during UAI (i.e., serosorting). Since familiarity between casual partners may lead to higher levels of UAI and serosorting, we examined how often men have UAI and practice serosorting with three types of casual partnerships that differ in their degree of familiarity. We included 240 HIV-negative men of the Amsterdam Cohort Study among MSM. We distinguished three types of casual partnerships: one-night stand ("met by chance and had sex only once"); multiple-time casual partner ("met and had sex with several times") and the "regular" casual partner ("sex buddy"). Serosorting was defined as UAI with an HIV-concordant partner. Generalised estimating equations analyses were used to examine the association between type of casual partnership and sexual risk behaviour. Analyses revealed that men with a sex buddy were more likely to have UAI than men with a one-night stand (OR [95%CI] 2.39 [1.39-4.09]). However, men with a sex buddy were also more likely to practice serosorting than men with a one-night stand (OR [95%CI] 5.20 [1.20-22.52]). Men with a sex buddy had more UAI but also reported more serosorting than men with a one-night stand. As a result, the proportion of UAI without serosorting is lower for men with a sex buddy, and therefore men might have less UAI at risk for HIV with this partner type. However, the protective value of serosorting with a sex buddy against HIV transmission needs to be further established. At this time, we suggest that a distinction between the one-night stand and the sex buddy should be incorporated in future studies as men behave significantly different with the two partner types.