The possible role of cows milk intolerance in the aetiology of infant colic was evaluated in 19 babies presenting to their health visitor or general practitioner in one town. Over a three week period a standard modified cows milk formula was compared with a soya milk formula on a double blind basis. The duration of colic symptoms was significantly reduced during the week on soya milk (P less than 0.01), with 11 out of 19 babies fulfilling the diagnostic criteria for cows milk intolerance. Four babies whose symptoms failed to improve either spontaneously or with soya milk were given a hydrolysed protein milk with a positive response in two, confirmed by challenge testing. Thus in 13 out of 19 babies (68%) the symptoms of infant colic resolved almost completely with dietary change.