We used a cluster analysis to empirically address whether sexual orientation is a continuum or can usefully be divided into categories such as heterosexual, homosexual, and bisexual using scores on the Klein Sexual Orientation Grid (KSOG) in three samples: groups of men and women recruited through bisexual groups and the Internet (Main Study men; Main Study women), and men recruited for a clinical study of HIV and the nervous system (HIV Study men). A five-cluster classification was chosen for the Main Study men (n = 212), a four-cluster classification for the Main Study women (n = 120), and a five-cluster classification for the HIV Study men (n = 620). We calculated means and standard deviations of these 14 clusters on the 21 variables composing the KSOG. Generally, the KSOG's overtly erotic items (Sexual Fantasies, Sexual Behavior, and Sexual Attraction), as well as the Self Identification items, tended to be more uniform within groups than the more social items were (Emotional Preference, Socialize with, and Lifestyle). The result is a set of objectively identified subgroups of bisexual men and women along with characterizations of the extent to which their KSOG scores describe and differentiate them. The Bisexual group identified by the cluster analysis of the HIV sample was distinctly different from any of the bisexual groups identified by the clustering process in the Main Sample. Simply put, the HIV sample's bisexuality is not like bisexuality in general, and attempts to generalize (even cautiously) from this clinical Bisexual group to a larger population would be doomed to failure. This underscores the importance of recruiting non-clinical samples if one wants insight into the nature of bisexuality in the population at large. Although the importance of non-clinical sampling in studies of sexual orientation has been widely and justly asserted, it has rarely been demonstrated by direct comparisons of the type conducted in the present study.