OBJECTIVE: To obtain the best estimate of the size of the effect of a thermoplastic resting splint on spasticity in the stroke-affected upper limb. DESIGN: A randomized controlled intervention involving 14 adults affected by stroke,allocated to two groups. SETTING: Inpatient and outpatient rehabilitation departments. INTERVENTION: Following one week of baseline when neither group wore a splint, group 1 continued without a splint for week 2 and then wore a splint during week 3. Group 2 wore a splint during weeks 2 and 3. Both groups then wore a splint through weeks 4-7. MAIN MEASURES: A computerized torque apparatus was used to measure resistance at the wrist in newtons at every one-degree angle through the range of extension. Amount and rate of change in resistance was compared between the groups to obtain the best estimate of the size of the effect of splinting. RESULTS: Effect sizes were small and failed to reach the suggested smallest clinically worthwhile effect size for amount and rate of change in resistance in the short term. However, the average estimated size of the effect for rate of change with longer term splinting exceeded the smallest clinically worthwhile effect. CONCLUSIONS: These findings and the fact that confidence intervals overlapped the smallest clinically worthwhile size of the effect for amount and rate of change in both short and long term suggest that a study with a larger sample is warranted.