- Background and purpose. The motor behaviour of children with cerebral palsy (CP) can be viewed in terms of a stable mode with very little flexibility that prevents adaptation to tasks. We hypothesized that the use of random perturbations (RP) would weaken excessive stability, introduce flexibility and enhance the effects of physical treatment. The objective was to evaluate the contribution of RP to gross motor function and mechanical efficiency (MEg) during intensive physiotherapy in children with CP. Methods. A convenience sample of 20 children with CP (mean age 8.2, range: 5.9‐12.9 yrs) were matched by age and GMFCS level, and randomly assigned to structured intensive treatment (SIT) or to SIT þRP groups. Groups received one month of daily treatment. RP was applied by engine-induced random passive cycling for upper and lower limbs for up to 10 min in a 90-min treatment session. Gross Motor Function Measure (GMFM)-66 and gross mechanical efficiency (MEg) during stair climbing (MEg) were measured before and after treatment. Results. GMFM-66 scores increased by about 1.0 in both groups. However, external work and MEg increased significantly more in SIT þRP than SIT. The increase in MEg in SIT þRP was independent of the level of motor function at baseline. Conclusion. The addition of RP in treatment of children with CP may have weakened previously established stereotypical motor patterns and introduced flexibility, thereby improving mechanical efficiency of a complex motor task. RP may enhance the effects of intensive treatment.