Lactose intolerance is a common adverse reaction to milk in adults, while milk hypersensitivity is a disorder of infancy. We hypothesized that milk hypersensitivity may cause many unspecific gastrointestinal disorders in adults. Twenty adults were subjected to double-blind, placebo-controlled milk challenge. Phagocyte activity, and Fc gamma and complement receptor expression of phagocytes were assayed, and serum total IgE, milk-specific IgE, and serum reactivity to milk protein were determined. The challenge increased phagocyte activity and complement receptor expression of phagocytes in subjects designated milk-hypersensitive, who had gastrointestinal symptoms from milk ingestion but normal lactose tolerance. The increase was not detected in lactose-intolerant or control subjects. The milk-hypersensitive group was also distinguished from the lactose-intolerant group by enhanced serum reactivity to milk protein. Only two out of nine milk-hypersensitive subjects had detectable milk-specific serum IgE. It is concluded that milk hypersensitivity in adults, occurring as gastrointestinal reactions, may be more common than previously thought.