Maximal aerobic capacity in bicycle ergometer tests was studied in nine children (aged 11 to 12 years) and five young men (aged 19 to 23 years), all with a spastic form of cerebral palsy, and compared with non-handicapped control groups. Results showed somewhat lower values for heart rate, oxygen uptake/kg, ventilation/kg and blood lactate concentrations for the majority of the spastic group. The physical work capacity of the handicapped group amounted to about 50 per cent of the corresponding values for the non-handicapped controls. Poor mechanical efficiency in the spastic group is the consequence of high oxygen uptake in relation to the work performed. The poor mechanical performance is mainly caused by the extra amount of energy required for qualitative changes in the muscle (constant hypertonia), involuntary movements and stabilizing movements during exercise on the bicycle ergometer.