Problematic hip subluxation is uncommon in spastic diplegia. We have identified three cases of late hip subluxation in teenage children with spastic diplegia associated with unrecognized hydrocephalus or ventriculoperitoneal shunt dysfunction. Each was a community ambulator with radiologically stable hips in childhood, but in adolescence underwent a deterioration in ambulatory status associated with rapid hip subluxation. Subsequent investigation revealed undiagnosed hydrocephalus in one patient and shunt dysfunction requiring revision in the other two. Surgery was performed in all three cases to control the hip subluxation. The atypical presentation of a progressively unstable hip in an ambulatory spastic diplegic patient may indicate the presence of previously unrecognized raised intraventricular pressure.