For two years after surgical small-intestine duplication a 9-year-old boy with the short bowel syndrome had recurrent acidosis which caused severe failure to thrive. During the acidotic crises he had behavioural disorders, unsteady gait, indistinct speech, lid raising weakness with vision paresis and occasional somnolence. These signs disappeared after the aciduria had been treated with high doses of bicarbonate. D-lactic acidosis was finally diagnosed by simultaneously determining D-lactate (8.9 mmol/l [normal < 0.5]) and L-lactate (1.4 mmol/l [normal < 1.78]) during an episode of aciduria (pH 7.3, base excess -11.8 mmol/l). Further acidotic crises were prevented by a carbohydrate-modified diet, on which he gained 8 kg in one year.