A randomized placebo-controlled trial was carried out to investigate the efficacy of neuromuscular electrical stimulation (NMES) and threshold electrical stimulation (TES) in strengthening the quadriceps muscles of both legs in children with cerebral palsy (CP). Sixty children (38 males, 22 females; mean age 11y [SD 3y 6mo]; age range 5–16y) were randomized to one of the following groups: NMES (n=18), TES (n=20), or placebo (n=22). Clinical presentations were diplegia (n=55), quadriplegia (n=1), dystonia (n=1), ataxia (n=1), and non-classifiable CP (n=2). Thirty-four children walked unaided, 17 used posterior walkers, six used crutches, and the remaining three used sticks for mobility. Peak torque of the left and right quadriceps muscles, gross motor function, and impact of disability were assessed at baseline and end of treatment (16wks), and at a 6-week follow-up visit. No statistically significant difference was demonstrated between NMES or TES versus placebo for strength or function. Statistically significant differences were observed between NMES and TES versus placebo for impact of disability at the end of treatment, but only between TES and placebo at the 6-week follow-up. In conclusion, further evidence is required to show whether NMES and/or TES may be useful as an adjunct to therapy in ambulatory children with diplegia who find resistive strengthening programmes difficult.