The dye Patent Blue V (PBV) is increasingly used for staging procedures in operable breast cancer, but is reported to cause adverse reactions. The aim of this study was to present the clinical features and the results of follow-up examinations in patients with such reactions. We studied nine patients with hypersensitivity reactions to PBV between 1999 and 2006 who were identified through the Norwegian network for reporting and investigating allergic reactions during anesthesia. We observed incidences of 0.5% (7/1418) for all kinds of PBV reactions and 0.4% (5/1418) for anaphylaxis. Typical clinical features included: (i) cardiovascular and/or cutaneous symptoms, (ii) a delay in symptoms, compared to the time of dye injection, (iii) poor response to ephedrine and intravenous fluid, and (iv) need for adrenaline administration, sometimes prolonged, for circulatory stabilization. Cutaneous manifestations were noted in five of the seven patients with anaphylaxis and two additional patients without circulatory instability. During anaphylactic reactions, serum tryptase was increased in six patients and normal in one. Serum tryptase was normal in one patient with skin symptoms only. Skin prick tests to PBV were positive in all eight patients tested, including the two with skin manifestations only. The clinical features and the results of follow-up studies strongly suggest that these reactions are IgE mediated.