The predominant form of 5alpha-reductase (5aR) in human scalp is 5aR1. None the less, clinical studies have shown that finasteride, a selective inhibitor of 5aR2, decreases scalp dihydrotestosterone and promotes hair growth in men with androgenetic alopecia. Immunolocalization studies were thus carried out to examine 5aR isozyme distribution within scalp and, in particular, to determine whether 5aR2 might be associated with hair follicles. 5aR2 was localized using both a rabbit polyclonal and a mouse monoclonal antibody. 5aR1 was detected with a mouse monoclonal antibody. The specificity of these reagents was demonstrated both by immunofluorescence and Western blot analyses of COS cells overexpressing human 5aR1 or 5aR2. When cryosections of scalp from men with androgenetic alopecia were stained with antibody against 5aR2, using immunoperoxidase avidin-biotin complex methodology, immunostaining was observed in the inner layer of the outer root sheath and, in more proximal regions of the follicle, in the inner root sheath. Staining was also prominent in the infundibular region of the follicle, with less intense staining extending throughout the granular layer of the epidermis. Some staining was also seen in sebaceous ducts. Similar results were obtained with both the polyclonal and monoclonal 5aR2 antibodies. In contrast, in scalp cryosections stained with antibody to 5aR1, no immunostaining was observed within hair follicles. Intense staining for the type 1 isozyme was, however, detected within sebaceous glands. Our immunolocalization data suggest that the results seen in clinical trials of men with male pattern hair loss treated with finasteride may be due, at least in part, to local inhibition of 5aR2 within the hair follicle.