This retrospective study documents the diagnostic and therapeutic value of steroid injections in patients with carpal tunnel syndrome. Two hundred eighty-one injections in 233 patients were done by one surgeon who used the same technique. Adequate follow-up ranging from 6 to 45 months was obtained in 199 patients (222 wrists). Injection of the carpal tunnel is an effective, albeit usually transient, therapeutic modality. Eighty-one percent of the patients obtained good or complete relief lasting from 1 day to 45 months. In most of these, symptoms began to recur after about 2 to 4 months (average 3.3 months), but in only 46% were recurrent symptoms severe enough to warrant surgical treatment. Twenty of the patients injected had no recurrence of symptoms for periods ranging from 10 to 45 months. Results of this study suggest that carpal tunnel injection is also a reasonably accurate diagnostic test. Ninety-nine wrists in 89 patients were subsequently treated surgically. Correlations between results of injections and subsequent operations indicate that a good response to injection is an excellent diagnostic and prognostic sign. However, the converse is not true; poor relief from injection does not necessarily mean that the patient is a poor candidate for surgery.