Hypothesis: The assumption that good performance on dexterity and strength measures is correlated with use of the new thumb after index pollicization for congenital thumb hypoplasia has not previously been tested. The Thumb Grasp and Pinch assessment (T-GAP) is a new measure of thumb use that classifies grasp patterns used by children after index pollicization in age-appropriate activities of daily living. We hypothesize that thumb use and hand dexterity/strength are related but not equivalent; therefore, we hypothesize low to moderate correlation between T-GAP scores and standard dexterity and strength outcome measures. Methods: Prospectively collected data from children with congenital thumb hypoplasia treated with index pollicization was reviewed. Standard outcomes measures included strength, range of motion, the Box and Blocks test (BBT), the Nine Hole Peg test (9HP), and the Functional Dexterity Test (FDT). Patients also completed the T-GAP consisting of 9 age-appropriate tasks designed to elicit specific hand and thumb use patterns. Grasp and pinch style were scored as follows: Palmar grasp without thumb (1 point); scissors between two most ulnar digits (2 points); scissors between radial digits (no thumb; 3 points); palmar grasp with thumb (4 points); key pinch (5 points); tip pinch (6 points); thumb to index and long (7 points). Scores for each task were summed to produce a final T-GAP score. Spearman correlation coefficients were calculated to describe the relationship between T-GAP scores and standard outcomes measures. Results: Twenty-two patients were included in the study. T-GAP score was significantly correlated with scores on the BBT (R ¼ 0.59), NHPT (R ¼ 0.66), and FDT (R ¼ -.70) (P < 0.02 for all, Table 1). T-GAP score was also significantly correlated with tripod pinch, key pinch and grip strength (R ¼ 0.75, 0.51, and 0.55 respectively) and with opposition and grasp span (R ¼ 0.59 and 0.70) (P < 0.05 for all, Table 2). Summary Points: The T-GAP effectively measures the complex active use of the pollicized digit during activities which require a variety of grasp and pinch styles. T-GAP score was correlated with strength, range of motion, and all 3 dexterity measures (BBT, 9HP, FDT). These correlations provide evidence for concurrent validity and construct validity of the T-GAP. Intermediate correlations imply that the T-GAP and standard dexterity tests measure related but distinct aspects of dexterity. The varied grasp and pinch styles employed by children with congenital thumb hypoplasia are not entirely captured by standard dexterity outcome measures, which are based on speed and allow any pinch pattern to be used, including those without use of the thumb.