Breast feeding has been recommended for the dietary treatment of infants with organic acidaemias, but studies documenting clinical experience are still very few. Nine infants, diagnosed with methylmalonic acidaemia (n = 4), propionic acidaemia (n = 1), isovaleric acidaemia (n = 2) and glutaric acidaemia type I (n = 2) were breast fed after diagnosis. The age of the patients was 28.9 +/- 13.4 months (mean +/- SD) (range 10-57 months). Eight patients were diagnosed with clinical symptoms and one because of an affected sibling. After the control of acute metabolic problems, an initial period with a measured volume of expressed breast milk was continued with on-demand breast feeding with the addition of a special essential amino acid mixture and energy supplements. Breast feeding was well tolerated in seven infants with good growth, metabolic control and neurological outcome. The duration of breast feeding was 12.3 +/- 7.4 months (mean +/- SD) (range 4-24 months) in these patients. Breast feeding was terminated in the patient with propionic acidaemia because of two acute metabolic episodes requiring hospitalization, and could not be continued in one of the patients with isovaleric acidaemia owing to shortage of breast milk. A decrease in the frequency of infections, acute metabolic episodes and hospital admissions was observed in breast-fed infants. Breast feeding of infants with organic acidaemias is feasible with close monitoring of clinical parameters such as growth, development and biochemistry, including amino acids, organic acids and ammonia.