MRI-guided transurethral ultrasound therapy offers a minimally invasive approach for the treatment of localized prostate cancer. The main goal of this study was to evaluate active temperature feedback on a clinical 1.5T MR imager to control conformal thermal therapy. MR thermometry was performed during heating in both thermal gel phantoms and ex-vivo tissue with a single-element transurethral heating applicator. The applicator rotation rate and power were controlled based on MRI-temperature measurements. The influence of a cooling gradient (to simulate cooling of the rectum or urethra) was also investigated in gel phantoms. The 55°C isotherm generated during heating closely matched the targeted prostate shape, with an average distance error of 0.9 mm ± 0.4 mm in turkey breasts, 1.3 mm ± 0.5 mm in gel phantoms without rectal cooling and 1.4 mm ± 0.6 mm in gel phantoms with rectal cooling. Accurate, MRI-guided, active feedback has been successfully demonstrated experimentally and has the capability to adjust for unpredictable and varying tissue properties during the treatment.