Fifty cases of untreated human T-cell lymphotropic virus type I (HTLV-I) associated myelopathy (HAM) were evaluated by urodynamic studies to clarify the nature of urinary disturbance and to find out suitable urological treatment. Both irritative and obstructive symptoms coexisted in the HAM patients. 38% of the patients experienced only urinary symptoms throughout the affected period. A main cause of frequency was detrusor hyperreflexia at filling phase, which was found in 58% of the patients. However, decreased effective bladder capacity due to large amount of residual urine was possibly an another cause of frequency. Detrusor sphincter dyssynergia was the main cause of difficulty of urination, but in some cases underactive detrusor at voiding phase was also the cause of difficulty of urination. Hydronephrosis was observed in only 5 kidneys, although as many as 30 out of 46 cases (65.2%) showed bladder deformity. 17 cases (34%) had urinary tract infection at first visit. As the activity of daily living was deteriorated, the mean volume of residual urine, incidence of detrusor hyperreflexia and detrusor sphincter dyssynergia were all increased. Medical treatment was effective to relieve subjective symptoms, but urodynamic examination did not necessarily review improvement. Intermittent catheterization was needed and successful in 64% of all cases. Patients with HAM must be carefully followed up by urologists.