Patent blue V is widely used in the identification of sentinel lymph nodes in patients with breast cancer and other malignancies. Individual case reports of allergy to patent blue V have been described in the medical literature since the 1960s. However, there is little data on clinical features and little experience of which allergy tests are appropriate or useful. We gathered all cases of patent blue V allergy that had been seen and diagnosed in the Department of Allergy, Addenbrooke's Hospital over a 3-year period. We collected clinical details of each case including skin test results. For comparison we recruited 12 healthy control subjects who then underwent skin testing to patent blue V. Six cases of patent blue V allergy were identified, all occurring during sentinel lymph node identification for breast carcinoma. All 6 had positive skin prick tests to neat patent blue V (25 mg/ml). Skin prick testing with a 1 : 10 dilution (2.5 mg/ml) produced positive results in 3 of 4 patients tested, and intradermal testing at a 1 : 100 dilution was (0.25 mg/ml) was positive in all patients tested. Of 12 control subjects, 11 had negative skin prick tests to both neat and 1 : 10 patent blue V with one subject showing a positive reaction to the higher concentration only. On the basis of our experience of patent blue V allergy, we propose a diagnostic protocol that can be safely and reliably utilised in centres equipped for allergy testing.