Gastroesophageal reflux (GER) in infants can be secondary to food allergy. We have evaluated the frequency with which GER is associated with cow's milk protein allergy (CMPA) in infants < 1 yr old and tried to indicate the laboratory and instrumental examinations useful in diagnosing GER + CMPA. We studied 140 infants (60 M, 80 F), mean age 6.0 +/- 2.8 months. After 24-h esophageal pH-metry, esophageal endoscopy, and elimination diet, followed by a double-blind challenge, the patients were divided into four groups: primary GER, GER secondary to CMPA, CMPA without GER, and a control group with subjects suffering from neither GER nor CMPA. Thirty of 72 patients with GER were also suffering from CMPA. No differences were observed as regards age, sex, symptoms, and clinical or family history between patients with GER only and those with GER + CMPA. The immunological test most useful for GER + CMPA diagnosis was the IgG anti-beta-lactoglobulin assay: positive in 27/30 subjects with GER + CMPA and in 4/42 patients with GER only. We also observed a characteristic pattern of the pH-monitoring tracing in 26/30 patients with GER + CMPA but in none of the 42 patients with GER only. This consisted of a progressive, constant reduction in esophageal pH at the end of a feed, which continued up to the following feed, when pH rose steeply. We conclude that the evidence of this characteristic tracing and of a high IgG anti-beta-lactoglobulin value are specific and sensitive tests for GER + CMPA diagnosis.