The thumb carpometacarpal joint is a common site of osteoarthritis. It has been hypothesized that localized stress peaks on the dorso-radial and/or volar-ulnar regions of the articular surfaces of the trapezium and metacarpal lead to cartilage erosion and may be resonsible for the progression of the disease. The objective of this study was to determine the contact areas in this joint under the functional position of lateral (key) pinch, and in the extremes of joint range of motion. These contact areas were assessed relative to the observed sites of cartilage thinning. Eight female and five male human cadaver hands were tested in vitro with the thumb under a 25 N load in the lateral pinch position, and under small muscle loads (0-5 N) with the thumb in flexion, extension, abduction, adduction and neutral positions. Contact areas of the thumb carpometacarpal joint articular surfaces were determined for these positions using a stereophotogrammetric technique. The lateral pinch position produced contact areas predominantly on the central, volar, and volar-ulnar regions of the trapezium and the metacarpal. In three specimens, contact areas were distinctly separated between the dorso-radial and volar-ulnar regions, and in one male specimen contact occurred exclusively on the dorso- radial region of the trapezium. Using stereophotogrammetry, cartilage thickness maps were also determined for a subset of nine specimens. The volar-ulnar, ulnar, and dorso-radial regions of the trapezium were the most common sites of thin cartilage, and these may be sites of cartilage wear. The results of this study indicate that the lateral pinch position produces stresses in the same regions where cartilage thinning is observed, lending support to the hypothesis that high stresses can lead to osteoarthritis in this joint.