Morphological properties of skeletal muscle were compared between wrist flexors and extensors within the same children (n=8, six females, two males; age range 4 to 9y, median age 7y)with wrist muscle imbalance secondary to spastic cerebral palsy (CP). Five patients had hemiplegic CP, two diplegic CP, and one patient had tetraplegic CP. Muscle biopsies were taken during either tendon transfer or tendon lengthening procedures. Analyses included distribution of muscle fibre types, fibre sizes, and expression of developmental myosins. Extensor fibre area was significantly greater than flexor fibre area for type 2A fibres and type 2B fibres but not for type 1 fibres. Coefficient of variation (CV) of fibre size for all three fibre types was greater for flexors compared with extensors. The greatest CV was observed for the type 2A fibres in flexors (39.5 [3.6%]). A wide variation was observed for expression of developmental myosin with the magnitude of the expression being greater, but not statistically significant, in flexors compared with extensors (5.4/mm2 vs 0.53/mm2). These data demonstrate that significant secondary myopathy of wrist flexor muscles results from CP.