In order to evaluate the role of lactose malabsorption in children with recurrent abdominal pain, we performed a prospective controlled double-blinded study in 40 children with RAP of at least three months' duration. Children were studied for lactose malabsorption by breath hydrogen determinations after ingestion of lactose (2 gm/kg of body weight; maximum 50 gm). Lactose malabsorbers were retested with 12.5 gm lactose; lactose absorbers were retested with lactose for ability to produce hydrogen. All children underwent a dietary trial which included two lactose elimination periods. Although 12 children (30%) were lactose malabsorbers, only three malabsorbed part of the smaller, more physiologic, lactose load. Improvement rates of lactose malabsorbers and absorbers during lactose elimination were not significantly different as judged by their physicians and as determined by a 50% or more decrease in pain frequency. These results suggest that lactose malabsorption is of little importance in children with RAP.