Blue dyes such as Patent Blue V (PBV) have been used in medical procedures for decades, and in the United Kingdom they are routinely utilised in sentinel lymph node biopsy (SLNB) for staging the axilla in early breast cancer. However, it has long been recognised that such dyes are associated with anaphylaxis. It has recently been estimated in a prospective study that allergy to PBV occurs with a frequency of 0.9%. Since repeated SLNB (and therefore further exposure to PBV) is increasingly being advocated for the small proportion of patients who develop a local (in-breast) recurrence, and because anaphylaxis can be life-threatening, it is important that those individuals that are allergic to PBV are recognised on their first medical exposure. The measurement of serum mast-cell tryptase (MCT) and skin prick test (SPT) are used in the investigation of suspected anaphylaxis because positive results are supportive of type-1 mediated hypersensitivity. Here we report the clinical features, MCT results and SPT results that pertain to a series of four patients referred to our drug allergy clinic with suspected anaphylaxis following SLNB. We recommend that all patients that show clinical evidence of allergy following exposure to PBV are referred to a specialist drug allergy service for further evaluation to investigate the cause.