Objective: To determine if ingestion of 2 doses of milk-based dietary supplements produce gaseous symptoms in subjects who malabsorb lactose. Design Randomized, controlled, crossover trial. Subjects/setting: Ten community-based subjects who malabsorb lactose. Intervention: Ingestion of 2 standard servings of milk-based supplements (a powder reconstituted in fat-free milk or a ready-to-drink preparation) or low-lactose control preparations. Main outcome measures: Frequency of flatus passage and subjective impression of bloating, flatulence, and abdominal discomfort. Statistical analysis: Wilcoxon signed-rank test. Results: The high lactose content (27 g) of 2 servings of the powder-based supplement ingested without other food resulted in a marked increase in daily flatus passages from the basal level of 9.7±8.2 to 30±14 (mean±SD), and a significant increase in the subjects' perception of gas. In contrast, the lower lactose content (18.4 g) of 2 servings of a ready-to-drink supplement resulted in a flatus frequency of 17±10 (P=.14 vs baseline) and no significant increase in the perception of increased gas. Neither supplement resulted in a significant increase in bloating, abdominal pain, or diarrhea. The lactose content of the liquid supplement was reduced by 80% following overnight incubation with an over-the-counter lactase preparation. Applications/conclusions: Persons who malabsorb lactose should be aware that sizable increases in rectal gas commonly occur when milk-based powders reconstituted in milk are used as meal replacements. In contrast, gas problems probably will be minor following ingestion of 2 doses of a ready-to-drink, milk-based supplement. The lactose content of these supplements can be markedly reduced by overnight incubation with over-the-counter lactase preparations, and this manipulation should be beneficial for subjects troubled by the increased gas caused by the consumption of lactose-containing supplements.