Both idiopathic carpal tunnel syndrome (CTS) and Raynaud's phenomenon (RP) are common, and may have similar clinical symptoms. The degree of their coexistence is uncertain. We have examined 30 patients, who were diagnosed clinically and electromyographically as having idiopathic CTS, for the presence of RP using a cold provocation test with photoplethysmography. The patients' hands were exposed in water at 10 degrees C for five minutes. A total of 18 patients (60%) was found to have RP; this is much greater than would be expected from the prevalence in the general population. Raynaud's phenomenon should be considered when treating patients with CTS because of the possibility of coexistence and the similar symptoms of these two disorders.