Seroadaptive behaviors have been widely described as preventive strategies among men who have sex with men (MSM) and other populations worldwide. However, causal links between intentions to adopt seroadaptive behaviors and subsequent behavior have not been established. We conducted a longitudinal study of 732 MSM in San Francisco to assess consistency and adherence to multiple seroadaptive behaviors, abstinence and condom use, whether prior intentions predict future seroadaptive behaviors and the likelihood that observed behavioral patterns are the result of chance. Pure serosorting (i.e., having only HIV-negative partners) among HIV-negative MSM and seropositioning (i.e., assuming the receptive position during unprotected anal sex) among HIV-positive MSM were more common, more successfully adhered to and more strongly associated with prior intentions than consistent condom use. Seroconcordant partnerships occurred significantly more often than expected by chance, reducing the prevalence of serodiscordant partnerships. Having no sex was intended by the fewest MSM, yet half of HIV-positive MSM who abstained from sex at baseline also did so at 12 month follow-up. Nonetheless, no preventive strategy was consistently used by more than one-third of MSM overall and none was adhered to by more than half from baseline to follow-up. The effectiveness of seroadaptive strategies should be improved and used as efficacy endpoints in trials of behavioral prevention interventions.