The clinical patterns of adverse reactions to cows' milk were examined in 72 children with cows' milk hypersensitivity. Two main groups were found, according to the time of onset of the adverse reaction--immediate onset, within one hour of milk ingestion and delayed onset, after one hour. Children with immediate onset reactions usually had cutaneous manifestations, positive prick tests, raised IgE values, were atopic, and the reaction was provoked by only small amounts of milk. Children with delayed onset reactions usually had gastrointestinal manifestations; negative prick tests; normal IgE values; were not atopic; had a history of vomiting, diarrhoea, and colic in the first year of life; and a larger amount of milk was needed to provoke the adverse reaction. Placing affected children into one or other category should increase the reliability of interpreting milk prick tests and clinical findings.