Sixty tumors with a minimum of 1-year follow-up were treated with radiation and superficial microwave hyperthermia (915 MHz). The overall local control rate was 50% (30/60). The most important factor in outcome was appropriateness of the hyperthermia applicator. Tumors covered by at least the 25% iso-SAR contour achieved 65% local control versus 21% local control with less than 25% SAR coverage (p less than 0.01). Several measures of adequate minimum monitored tumor temperature and duration were considered. The measure best correlated with outcome was best single session time at or above 43 degrees C (t43). If each monitored tumor catheter achieved t43 greater than or equal to 30 minutes in at least one session, then tumor control was significantly (p less than 0.01) improved (63% with Min t43 greater than or equal to 30 versus 25% with Min t43 less than 30). Although there was considerable overlap between tumors with SAR greater than or equal to 25% and those achieving Min t43 greater than or equal to 30, a statistically significant (p = 0.02) difference could be demonstrated between the group meeting both the SAR and the minimum tumor time/duration standards as opposed to those meeting only one standard. The actuarial local progression-free survival for tumors most likely to have had adequate hyperthermia (defined as SAR greater than or equal to 25% and Min t43 greater than or equal to 30) and all other tumors did not begin to separate significantly until 8 to 12 months after treatment. Implications for future randomized studies are discussed.