Background: Little is known about how performance on strength, range of motion, and dexterity measures changes as children with index finger pollicization mature. The authors reviewed performance in range of motion, strength, and dexterity over a 7-year period and report outcomes over time. Methods: Data from children treated with index finger pollicization for congenital thumb hypoplasia from 2007 to 2014 were reviewed retrospectively. Children were followed for an average of 3.9 years (range, 1 to 7 years) during the study period. Standardized assessments included range of motion, grip, key pinch and tripod pinch strength, the Box and Block Test, the Nine Hole Peg Test, and the Functional Dexterity Test. Average score by age and average yearly change were calculated for each assessment, and scores were plotted against published age-matched scores of normal children when available. Results: Twenty-three patients with 29 affected thumbs were included. Distal grasp span increased 0.17 inch and Kapandji opposition improved 0.26 point with each year of age; however, proximal web-space size did not increase over time. Grip strength improved an average of 2.69 kg/year, and tripod and key pinch improved 0.58 kg and 0.67 kg with each year of age. Box and Block Test scores improved an average of 4.11 blocks/year. Scores on the Nine Hole Peg Test improved 3.83 seconds/year, and scores on the Functional Dexterity Test improved 0.026 peg/second each year. Conclusions: Children with pollicized thumbs improve in dexterity and strength with growth. Web-space size did not change with age; therefore, the thumb should be carefully positioned at the time of surgery.