Transurethral microwave thermotherapy (TUMT) is a single-session, minimally invasive outpatient treatment for patients with symptoms of benign prostatic bladder outflow obstruction. We designed a prospective randomised trial to identify any placebo response. Patients with a Madsen symptom score over 8 for at least 6 months were eligible for study. Two measurements of urinary flow less than 15 mL/s and a residual urine of under 350 mL were also required for entry. Patients with renal dysfunction, upper urinary tract disease, co-existing bladder disease, and malignant prostatic change were excluded. 43 patients were studied: 21 were randomised to receive a sham treatment and 22 to thermotherapy. Sham treatments were done with the urethral applicator in situ. 40 patients were available for evaluation at 3 months. 2 patients had delayed follow-up and 1 patient randomised to TUMT has undergone transurethral resection. The thermotherapy group showed a 70% decrease (from 14.5 to 4.3) in the mean Madsen score, a 53% increase in flow-rate (8.5 to 13.0 mL/s), and 92% decrease in residual urine volume (147 to 12 mL). No significant change was seen in these mean indices in the sham group. There was no difference in the main complication of transient haematuria between the two groups. However, there was a 22% frequency of acute retention in the TUMT group. The results show little significant placebo component to the subjective and objective improvement that occurs in patients who have received TUMT.