Clinical and laboratory observations were made with 38 children afflicted with chronic severe asthma (reversible obstructive airway disease) in which hypersensitivity to food was incriminated in the histories. Symptoms were evoked in double-blind food challenges in only 11/38 children and 14/70 challenges, and were characteristic of immediate-type hypersensitivity and were chiefly gastrointestinal, even though asthma was the common presenting complaint. There were no delayed reactions. Peanut was responsible for 8 reactions, egg for 5, and cow's milk for 1. The feature that most successfully identified those having positive reactions in challenges was a significant wheal reaction in a skin test by puncture technique using a verified extract of 1:20 W/V concentration. No subject with clinically significant, symptomatic hypersensitivity to food had a negative puncture test, and puncture tests were positive in only 10/56 instances of negative reactions in food challenges. Laboratory observations included release of histamine and enzymes from leukocytes and the levels of neutrophil enzymes in serum before and after food provocation tests. While these determinations were of interest with respect to the immunochemical basis of reactions to foods, they did not prove useful for practical clinical diagnosis. The outstanding laboratory findings was the occurrence of "spontaneous" release of 25% to 100% of the histamine from leukocytes in all cases proved clinically hypersensitive by food challenges, which suggests that this may be an indicator of immediate-type hypersensitivity to food. From the findings in the study, a general approach to food hypersensitivity was developed in which the immunologic components coupled with quantitative concentration-response relationships serve to render comprehensible the distinction between asymptomatic (immunologic) hypersensitivity and symptomatic (clinical) hypersensitivity.