Basal joint arthritis of the thumb and carpal tunnel syndrome are common conditions with an acknowledged coexistence. This article attempts to quantify the prevalence of carpal tunnel syndrome in patients with basal joint arthritis and to examine some of the etiologic factors that affect the coexistence of the two disorders. Of 246 patients who had surgery about the basal joint, 95 patients (39%) were identified by chart review as having carpal tunnel syndrome. Eleven of 122 remaining patients contacted had symptomatic carpal tunnel syndrome confirmed by nerve-conduction studies, bringing the total to 106 (43%). The prevalence was higher in worker's compensation patients and those with diabetes mellitus. The prevalence was lower in men than in women, and patients with inflammatory joint disease were at less risk than those with osteoarthritis. Given this high association, great care should be taken to diagnose or exclude coexistent carpal tunnel syndrome in patients scheduled for basal joint surgery so that, if present, it can be treated at the same time, diminishing the risk of postoperative morbidity and delayed symptoms.