Transient lactose intolerance has been identified as a possible causative factor in infant colic. A double-blind randomised placebo-controlled crossover study to investigate this has been undertaken in 53 babies with symptoms of colic. Pre-incubation of the feed with lactase resulted in breath hydrogen levels and total crying time which were both at least 45% lower than figures with placebo treatment, in 26% of the full trial group (95% confidence interval 12.9% to 44.4%), and in 38% of compliers (95% confidence interval 18.8% to 59.4%). The remainder did not respond to the same extent. These findings suggest that infant colic may have a multiple aetiology, and that in a significant number of cases the immediate cause is transient lactose intolerance, in which cases pretreatment of feeds with lactase can result in considerable symptomatic benefits.