Clinical, microbiological, and immunologic responses were evaluated in volunteers vaccinated intradermally with bacille Calmette-Guérin (BCG). Most volunteers (98%) developed ulcerative lesions that drained for a mean +/- SE of 4.3 +/- 0.29 weeks. Mycobacterial DNA was detected by a polymerase chain reaction-based amplification technique in biopsy specimens from BCG ulcers 2 weeks after vaccination and in blood specimens 3 days after vaccination. Mycobacteria were cultured from ulcer drainage 2 months after vaccination, demonstrating a prolonged potential risk of contact spread of the vaccine strain. The duration of ulcer drainage was inversely correlated with prevaccination lymphoproliferative (r = -0.515; P < .002) and interferon gamma (r = -0.841; P < .002) responses specific to mycobacteria and directly correlated with postvaccination increases in lymphoproliferative (r = 0.498; P < .002) and interferon gamma (r = 0.688; P < .02) responses specific to mycobacteria. These results demonstrate the clinical reactogenicity of BCG and the potential risk of contact spread of the vaccine strain and suggest that clinical reactogenicity is a trade-off for the induction of protective mycobacterial immunity.