Severe neurological symptoms, including intermittent ataxia, hallucinations and convulsions, associated with metabolic acidosis and branched-chain amino-acidemia occurred in a six-year-old girl with a variant form of maple syrup urine disease. The symptoms only appeared during periods of infection. Between these periods the girl was healthy and the biochemical findings were normal. In later episodes of infection the condition was successfully treated with a low protein diet and sodium bicarbonate. Analyses of 1-14C-leucine decarboxylase in fibroblasts revealed 10 per cent of normal activity in the girl and 50-70 per cent in the parents. The importance of early diagnosis of MSUD variants is discussed. This is the first published Swedish case of MSUD variant.