Between January 2001 and December 2003, 189 adult patients with balanoposthitis were treated in my clinic. Swab culture from the affected lesion detected Streptococcus pyogens in 47 cases (PYO group), other bacterial species including Candida albicans in 93 (B group), no bacterial growth in 27 (NB group) and swab culture was not done because clinical symptom was trivial in 22 (N group). The PYO group had a significantly higher prevalence of purulent discharge (68.1%) and local pain (38.3%), compared to the B group (25.8% and 21.5%, respectively) or the NB group (33.3% and 11.1%, respectively). Phimosis was absent in 7 cases in the PYO group. The route of infection in the PYO group was considered to be predominantly via sexual contact (PYO group 78.7%, B group 52.7%, NB group 59.3%), especially through fellatio by commercial sex worker for the majority of the PYO patients. The latent period (from sexual contact to the onset of symptoms) was 3 days or less in 40.5% and 4 to 7 days in 35.1% in the PYO group, whereas it was more than a week or not remembered in the majority of the B and NB groups, disapproving the causal relationship with sexual contact in such cases. Treatment with tosufloxacin tosilate or amoxicillin was effective in most cases of streptococcal balanoposthitis. Our results suggest that Streptococcus pyogens is one of the causative organisms of sexually transmitted balanoposthitis.