Background: The use of lupine in food has been increasing during the last decade and allergic reactions to lupine have been reported, especially in peanut-allergic patients. The frequency and the degree of cross-reactivity to other legumes are not known. The aim of the study was to investigate the frequency of sensitization to lupine, and in addition to pea and soy, and its clinical relevance, in peanut-sensitized patients. Furthermore, to determine the eliciting dose (ED) for lupine using double-blind placebo-controlled food challenges (DBPCFC). Methods: Thirty-nine unselected peanut-sensitized patients were evaluated by skin prick tests (SPT) and ImmunoCAP to lupine, pea, and soy. Clinical reactivity was measured by DBPCFC for lupine, and by history for pea and soy. Results: Eighty-two percent of the study population was sensitized to lupine, 55% to pea, and 87% to soy. Clinically relevant sensitization to lupine, pea, or soy occurred in 35%, 29%, and 33% respectively of the study population. None of the patients was aware of the use of lupine in food. The lowest ED for lupine, inducing mild subjective symptoms, was 0.5 mg, and the no observed adverse effect level (NOAEL) was 0.1 mg. No predictive factors for lupine allergy were found. Conclusion: In peanut-sensitized patients, clinically relevant sensitization to either lupine or to pea or soy occurs frequently. The ED for lupine is low (0.5 mg), which is only fivefold higher than for peanut. Patients are not aware of lupine allergy and the presence of lupine in food, indicating that education is important to build awareness. © 2008 Blackwell Munksgaard.