The goals of surgery in treating intractable epilepsy are to eliminate seizures and improve quality of life. This report describes the development of the Epilepsy Surgery Inventory (ESI)-55, a 55-item measure of health-related quality of life for epilepsy patients. The ESI-55 includes the following scales (number of items in parentheses): health perceptions (9), energy/fatigue (4), overall quality of life (2), social function (2), emotional well-being (5), cognitive function (5), physical function (10), pain (2), and three separate scales of role limitations due to emotional, physical, or memory problems (5 items each). Also included is one change in health item. The ESI-55 was completed by 89% of 224 adults who had undergone a protocol evaluation for epilepsy surgery since 1974. Alpha internal consistency reliability coefficients ranged from 0.76 to 0.88 except for social function (alpha = 0.68). Multitrait scaling analyses supported item discrimination across scales. Factor analysis confirmed previously identified mental and physical health factors, and yielded a third factor defined by cognitive function and role limitations scales. Construct validity was supported by correlations of the ESI-55 with a mood profile instrument. Analysis of ESI-55 scale scores by seizure classification showed that the 44 patients who were seizure-free following surgery scored higher than did 55 patients who continued to have seizures (P less than 0.05 for all comparisons); 43 patients having seizures without loss of consciousness scored in between. Results of this study indicate that the ESI-55 is reliable, valid, and sensitive to differences in seizure status.