The clinical utility of serum 3 alpha-androstanediol glucuronide level has been controversial. Among the concerns regarding its lack of utility has been the finding that suppression of serum 3 alpha-androstanediol glucuronide does not occur readily with treatment. We hypothesized that because the treatment of hirsutism requires a prolonged duration, a longer observation period is required for changes in serum 3 alpha-androstanediol glucuronide to be measured. Therefore, we studied the clinical and hormonal changes in 11 women treated for hirsutism with a gonadotropin-releasing hormone agonist (GnRH-a) for 1 year. A progressive reduction in Ferriman-Gallwey scores occurred, which was significant at 6 weeks and was maximal at 12 months. Serum 3 alpha-androstanediol glucuronide and another peripheral marker, androsterone glucuronide, also fell commensurately. While there was no correlation at 3 months, by 6 weeks a significant correlation had occurred between the suppression in Ferriman-Gallwey scores and the suppression of serum 3 alpha-androstanediol glucuronide and androsterone glucuronide. The suppression of these steroids also correlated with the suppression of non-sex hormone-binding globulin-bound testosterone. These data confirm that markers of peripheral androgen action, particularly serum 3 alpha-androstanediol glucuronide, reflect the clinical manifestation of hirsutism. However, it appears that modifications in peripheral androgen activity (presumably through 5 alpha-reductase activity) are time-dependent, and that serum markers reflect changes after 6 months of treatment.