A patient-based, subjective outcome measure for chronic anterior cruciate ligament deficiency was developed, pretested, and validated. The development of the outcome measure was completed in four stages: 1) item generation (167 items), 2) item reduction (by surveying 79 patients), 3) questionnaire formation and pretesting (20 patients), and 4) reliability, responsiveness, and validity assessment (100 patients). This disease-specific quality of life measure is a 32-item questionnaire using a 100-mm visual analog scale response format. Face validity was derived from extensive direct patient input. Content validity was determined by consensus among 20 knee surgeons. There was no significant difference on test-retest reliability, with an overall average error of 6%. The outcome measure demonstrated responsiveness to clinical change correctly in 21 of 25 patients (84%). Construct validity was shown by the ability of the questionnaire to measure the full spectrum of disease; scores ranged from 8 to 99 out of a maximum score of 100. The questionnaire was able to distinguish patients who went on to have surgery (average score, 31) from those who would be treated nonoperatively (average score, 79). The quality of life outcome measure for chronic anterior cruciate ligament deficiency is the first subjective knee scale of its kind to be validated to measure quality of life in patients with chronic anterior cruciate ligament deficiency.