This prospective randomized double-blind control trial compared lengthening and simple division of the flexor retinaculum in carpal tunnel decompression. Twenty-six patients with bilateral carpal tunnel syndrome were randomly allocated to have the flexor retinaculum divided on one side and lengthened on the other. All 52 hands were reviewed at regular intervals up to 25 weeks. The patients, therapists and the final reviewer were unaware of treatment allocation. The Levine symptom and function scores were used to assess the severity of the carpal tunnel syndrome and showed that the two treatments were comparable for relief of carpal tunnel symptoms. The two treatments were also similar for function measured with the Jebsen-Taylor test. There is no identifiable benefit in lengthening the flexor retinaculum when decompressing the carpal tunnel. Moderate or severe pillar and scar pain is common, occurring in 13 of 52 hands after surgery, but only in four by the 12th week and two by the 25th week.