Fifty-seven children with cerebral palsy (CP) and imaging evidence of vascular thrombosis (study group) and 167 children with CP and other imaging finds (control group)were selected. Sixty-one per cent of the study group were male and 53 (93%) had spastic hemiplegia compared with the control group, of whom 55% were male and 54 (32%) had a diagnosis of spastic hemiplegia. Mean age was 5 years 11 months (SD 5y 1mo) for the study group and 7 years 7 months (SD 4y 7mo) for the control group. Blood spots on Guthrie cards or buccal swabs were used to test both groups and their mothers for the factor V Leiden (fVL) mutation, which predisposes carriers to thrombophilia. Mothers were interviewed to gather antenatal, perinatal, demographic, and socio-economic data. The frequency of the fVL mutation in children with evidence of vascular thrombosis and their mothers was not statistically different from the frequency in children with CP with other imaging findings and their mothers. The frequency of the fVL mutation was significantly higher than the expected population frequency of 4% in the study group (10.5%, p=0.012) and in mothers of the control group (7.2%, p=0.036).