On the first day of life, a healthy infant was given a recombinant hepatitis B vaccine. Over the following year, a 3 by 4.5 cm, well-defined, erythematous patch with an overlying white, reticulated, smaller plaque gradually appeared on her thigh at the vaccination site. Darier's sign was elicited at the site. Examination of a biopsy specimen showed an upper dermal mast cell infiltrate. This is the first reported case of a solitary mastocytoma appearing in a vaccination site.