We studied, retrospectively, 131 patients who had undergone an open operation for the carpal tunnel syndrome (CTS) in 229 hands. The symptoms were present on both sides in 59% of patients when first seen. Neurophysiological impairment of the median nerve was observed in 66% of the asymptomatic hands, and 73% of patients in this group developed symptoms of CTS after the opposite side had been operated on. Follow-up of patients with unilateral CTS showed that the subsequent development of disease in the unaffected hand is very common. We conclude that CTS is a bilateral disorder and that it becomes more evident as time passes. There is a correlation between the duration of symptoms and bilateral occurrence.