An AB single-subject research design was used to assess the effectiveness of a short thumb opponens splint on hand function in a 4-year-old girl with cerebral palsy. Baseline data for active range of motion, grip and pinch strength, grasp patterns, the Box and Block Test of manual dexterity (Mathiowetz, Voland, Kashman, & Weber, 1985), and 1-in. cube stacking were collected twice a week for 4 weeks. The child was fitted with a short thumb opponens splint, which was worn 6 hr during the day and all night for 4 weeks. The twice-weekly measures of the dependent variables continued during the treatment phase. Visual and statistical analysis of the data indicate that the child showed a clinically significant improvement in palmar and radial abduction, thumb opposition, grip strength, performance on the Box and Block Test scores, cube stacking, and lateral pinch. These results suggest that for this child with cerebral palsy, the use of a short thumb opponens splint improved underlying aspects of hand function as well as hand function itself. Replication of this study with a more complex single-subject design involving more subjects is recommended to confirm these results.