The association of lactase deficiency with recurrent abdominal pain was investigated. One hundred three white children between the ages of 6 to 14 years with recurrent abdominal pain were evaluated. Sixty-nine underwent lactose tolerance tests and 26 had intestinal biopsies with lactase determinations; 21 of 69 (30.4%) had abnormal lactose tolerance tests and eight of 26 (31%) were lactase deficient. However, 16 of 61 (26.4%) control subjects matched for age and ethnic background exhibited lactase deficiency. Thus, a similar prevalence of lactase deficiency was found in the control and the recurrent abdominal pain groups. Thirty-eight patients with recurrent abdominal pain completed three successive six-week diet trials conducted in a double-blind fashion. An increase above base line value in pain frequency was seen in ten of 21 (48%) lactose malabsorbers and four of 17 (24%) lactose absorbers. After a 12-month milk elimination diet, six of 15 (40%) malabsorbers and five of 13 (38%) absorbers had elimination of their pain. This result compared with improvement occurring in five of 12 (42%) absorbers with recurrent abdominal pain who received a regular diet for one year and suggests that the elimination of lactose will not affect the overall frequency of improvement in recurrent abdominal pain. In addition, the recovery rate from recurrent abdominal pain is similar in both lactose absorbers and nonabsorbers independent of dietary restrictions.