Breath hydrogen excretion as an index of incomplete lactose absorption was measured in 118 healthy infants who were either breast fed or given a formula feed containing lactose, some of whom had colic. Infants with colic (n = 65) were selected on the basis of the mother's report of a history of inconsolable crying lasting several hours each day. Infants in the control group (n = 53) were not reported to cry excessively by their mothers. Breath samples were collected using a face mask sampling device preprandially, and 90 and 150 minutes after the start of a feed. Normalised breath hydrogen concentrations were higher in the group with colic than in the control group at each time point. The median maximum breath hydrogen concentration in the colic group was 29 ppm, and in the control group 11 ppm. The percentage of infants with incomplete lactose absorption (breath hydrogen concentration more than 20 ppm) in the colic group was 62% compared with 32% in the control group. The clinical importance of the observed association between increased breath hydrogen excretion and infantile colic remains to be determined. Increased breath hydrogen excretion indicative of incomplete lactose absorption may be either a cause or an effect of colic in infants.