Muscle function often becomes progressively more compromised in children with spastic cerebral palsy, leading to reduced mobility. This study aimed to examine the role that muscle connective tissue plays in this process. Severity of spasticity as determined by a range of clinical measures was assessed in 26 children (14 males 12 females; age range 4 to 17 years) with either diplegic or quadriplegic cerebral palsy (CP). Muscle biopsies from the vastus laeralis muscle were obtained for biomedical and histological analysis during orthopaedic surgery as part of the child's ongoing care. Total collagen was quantified by hydroxyproline determination. Two clinical measures of severity, Modified Ashworth Scale and Balance, were shown to have a highly significant correlation with collagen content, and Ambulatory Status, Clonus, and Selective Muscle Control all showed positive trends. Collagen I accumulated in spastic muscle's endomysium which appeared to be thickened, and fibrotic regions with sparse muscle fibres were evident in more severe cases. This suggests that collagen may be involved in increases in muscle stiffness observed in spasticity. Once developed, these changes are essentially irreversible and we suggest that future treatments should consider including prevention of muscle fibrosis.