Twenty-nine patients (16 males, 13 females) with Joubert syndrome were identified from ophthalmology, neurology, and genetic databases covering a 15-year period at Great Ormond Street Hospital, London. Criteria for diagnosis included absent or markedly hypoplastic cerebellar vermis, abnormal eye movements, and developmental delay. Five patients had died. Scans and notes were available for 22 patients, and 18 cases were clinically reviewed. The median age was 10 years 10 months (range 3mo to 19y) and the median follow-up was 8 years 5 months (range 3mo to 19y, with one new patient seen at 3mo of age). Cerebellar vermis hypoplasia/aplasia with 'molar tooth sign' in the axial plane was present in 22 of 22 patients, coloboma in 6 of 22, and polydactyly in 6 of 22. In the 18 clinically reviewed, apnoea occurred in 13 patients. Five had renal problems with cysts and 4 of 5 had abnormal electroretinograms (ERGs). Visual electrophysiology was abnormal in 14 of 18 patients, and in 6 there was evidence of deterioration in the ERG. Blood investigations of organic acids, phytanic acid, very-long-chain fatty acid, and transferrin were normal in 12 patients tested. Developmental assessment showed that 6 of 15 patients aged more than 5 years were at mainstream school, and 12 of 18 had started walking between 22 months and 10 years. Speech difficulties and behavioural problems were prominent.