A total of 100 patients treated with a single session of microwave thermotherapy at 4 European centers was stratified according to 2 different types of obstruction (constrictive and compressive) and compared to clinical outcome at 6 months. Patients had a Madsen-Iversen score of 8 or more, maximum flow rate of 15 ml. per second or less and residual urine volume of 300 ml. or less at entry. The change in Madsen-Iversen score was the same in the 2 groups. Maximum flow rate increased from 8.71 plus/minus 2.62 to 14.73 plus/minus 4.04 ml. per second in the constrictive group, and from 8.54 plus/minus 2.26 to 10.41 plus/minus 4.52 in the compressive group (p less than or equal to 0.0001). Residual urine decreased from 96.00 plus/minus 72.85 to 40.34 plus/minus 56.33 ml. in the constrictive group and from 109.86 plus/minus 67.09 to 84.65 plus/minus 81.45 ml. in the compressive group (p less than or equal to 0.0001). Success, as defined by an increase of 50 percent or more in maximum flow rate and Madsen-Iversen score, was noted in 68 percent of the constrictive but only 15 percent of the compressive groups (p less than or equal to 0.0001 chi-square test for trend). Selection by pressure-flow criteria for patients being considered for thermotherapy should improve the overall clinical results.