The development and testing of the Well-being Index for Surgical Patients (WISP), an instrument designed to assess the well-being of patients between 1 and 4 months after abdominal surgery, is described. The measure was intended for use in a study of the effectiveness of a dietary supplement for patients undergoing such surgery. Interviews were conducted with 15 patients who had had abdominal operations between 1 and 4 months previously. They provided information about their well-being at the time of their interview and also during the period since they had been discharged from hospital. A draft questionnaire was derived from the interviews and field tested with a further 17 patients who found the content to be relevant and the questionnaire easy to complete. A further 44 patients completed the measure on three occasions; 1, 2 and 4 months after abdominal surgery. The instrument was shown to have good internal consistency and produced statistically significant improvements in well-being over time. A postal survey with 20 patients indicated that the instrument's test-retest reliability was adequate. The WISP is acceptable to patients and easy to administer, making it suitable for inclusion in clinical trials.