Minocycline hydrochloride as a sclerosant for the treatment of simple renal cysts was evaluated. Cyst puncture was performed, and minocycline hydrochloride solution for intravenous administration was injected after aspiration. Of 154 cysts evaluated by ultrasound after 3 months or more, 69 were no longer demonstrable, 49 showed 50% or greater reduction in the maximum cyst diameter, and only 8 were unchanged or slightly increased in size. In constrast, only 1 of 20 cysts aspirated without minocycline hydrochloride injection (controls) showed 50% or greater reduction after 3 months. There was a significant difference in the reduction rates between the minocycline-treated group and control group (p<0.01). These results suggest that minocycline hydrochloride is an effective sclerosant to treat simple renal cysts.