Neurological abnormalities are common in patients with organic acidaemias and sometimes these are distinctive. Lesions in various parts of the brain are increasingly being recognised in patients with methylmalonic acidaemia. We present the cases of two patients with methylmalonic acidaemia who became acutely ill with marked metabolic acidosis. CT scan showed bilateral areas of low density involving the globi pallidi. One patient developed dystonic posturing of the upper and lower limbs with rigidity and bradykinesia following her recovery. Treatment with benzhexol and with levodopa and carbidopa had no sustained benefit. Her gait slowly improved but her dystonic posturing remained. The other patient developed marked truncal hypotonia and a variable increase in limb tone following the acute illness. These abnormalities have persisted. It is concluded that structural brain changes, and in particular symmetrical lesions in the globi pallidi, may follow acute illness in patients with methylmalonic acidaemia and be accompanied by neurological symptoms.