This study investigated determinants of the subjective effects of marijuana and the relationship of subjective effects to marijuana use. Subjects were 8169 twins drawn from the Vietnam Era Twin Registry. Subjects who used marijuana more than five times (n = 2513) reported whether they experienced each of 23 subjective reactions. Factor analysis identified a positive (pleasant) reaction factor and a negative (unpleasant) reaction factor. Both factors were related to duration and frequency of use. Pairs in which both members used marijuana more than five times (MZ = 352 pairs; DZ = 255 pairs) were examined to assess determinants of subjective effects. Approximately one-quarter of the variance in each factor was determined by additive genetic influences; the remaining variance was determined by environmental factors that are not shared by members of a twin pair. The shared or family environment had no detectable influence on either subjective reaction factor.