AIM: To evaluate the effect of individualized resting mitt splints on hypertonicity (spasticity and tissue stiffness) and passive range of motion (PROM). METHODS: A randomized, single blinded, single case design. Ten adults with acquired brain injury were randomized to control (no-splint) and experimental (splint) groups. The experimental group received an individualized (wrist position, wearing schedule) thermoplastic resting mitt splint. Measures included wrist and finger PROM, muscle stiffness (Modified Ashworth Scale), and spasticity (Modified Tardieu Scale) which were taken at five time points. RESULTS: Between-group analyses indicated a statistically significant effect on PROM at the wrist (d = 2.14, CI₉₅ = 0.57, 3.72, p < 0.05) and clinically important effects on finger PROM, and wrist and finger spasticity and stiffness. Within-group analyses indicated that splint-wear resulted in positive clinical effects ranging from zero effect (maintenance of pre-splinting status) to a large positive treatment effect. Non splint-wear resulted in negative clinical effects ranging from zero effect to a large negative treatment effect. CONCLUSION: Individualized resting splints for adults with moderate hypertonicity and no soft tissue contracture resulted in positive clinical effects to PROM, muscle stiffness and spasticity. Long-term splint-wear may be more beneficial than short-term wear, and may prevent the negative changes evident with no splint-wear. Resting hand splints should be considered for a select group where reduction in muscle stiffness and spasticity, or maintenance of PROM, is desired.