The purpose of this study was to evaluate effects of a progressive strength training programme on walking ability in adults with cerebral palsy. Ten individuals with spastic diplegia (seven males, three females; mean age 31, range 23–44 years) participated twice a week over 10 weeks. Seven individuals with spastic diplegia (four males, three females; mean age 33, range 25–47 years) who did not receive strength training served as controls. All individuals were ambulatory but motor ability ranged from functional walkers to individuals who always required walking aids and used a wheelchair regularly. Significant improvements were seen in isometric strength (hip extensors p=0.006, hip abductors p=0.01), and in isokinetic concentric work at 30°/s (knee extensors p=0.02) but not in eccentric work. Results also showed significant improvements in Gross Motor Function Measure (GMFM) dimensions D and E (p=0.005), walking velocity (p=0.005), and Timed Up and Go (p=0.01). There was no increase in spasticity for those who underwent strength training. Individuals in the control group did not show any significant improvement in any measured variable. The groups were small, however, and there was no significant difference between the groups in any measured variable. These findings suggest that a 10-week progressive strength training programme improves muscle strength and walking ability without increasing spasticity.