We report a 15-month-old boy who developed an ulcer in the left axillary fold following bacillus Calmette-Guerin vaccination. Subsequent immunologic and genetic studies led to the diagnosis of chronic granulomatous disease. His mother had "lupus-like" lesions, described in some carriers of this disease, that were thus related to her son's diagnosis. Although in healthy subjects this vaccination is usually harmless, in instances of impaired immunity it may cause adverse reactions. When a vaccine-related complication occurs, an underlying immunodeficiency should be sought.