The surgical findings in 147 consecutive hands that underwent primary carpal tunnel release between 1983 and 1993 were reviewed for anatomic variation. A classification is introduced for the two major types of variations found. The variations found within the carpal tunnel, termed intrinsic or Type I, were space-occupying lesions of muscular, bony, or varied soft tissue origin. The variations overlying the transverse carpal ligament, termed extrinsic or Type II, were nearly all muscular in origin. Seventy-four variations were found in 60 hands. Overall, an underlying variation was found in 41% of the hands. After applying a logistic model, it was found that patient age and hand dominance were important factors in predicting the presence of an anatomic variation. For patients 40 years of age or younger, the odds of observing an anatomic variation were 3.2 times higher than the odds of observing a variation in patients older than 40 years. The odds of observing an anatomic variation in the dominant hand were 2.3 times higher when compared with the odds of observing a variation in the nondominant hand. In patients 40 years of age or younger, with dominant side involvement, an anatomic variation was found in 71% of hands. In patients older than 40 years with nondominant side involvement, an anatomic variation was found in 24% of hands. Taking into account not only anatomic variations but also underlying medical disease and local injury, at least one was identified in 86% of hands.