More than 300 patients with neoplastic disease (most with lymphoma or leukemia) were given one or more BCG vaccinations. In the great majority of cases, these were well tolerated. The complications we observed were of three types: (A) persistent BCG infection that could disseminate widely, (B) activation of old, dormant acid-fast infection, and (C) hypersensitivity reactions. The latter were related to the frequency of administration and dose of organisms. It is important to recognize that BCG organisms may not be completely destroyed by patients with impaired immunologic defenses, and that their inoculation may create a source of disseminated acid-fast infection at a later time. In our limited experience, treatment with isoniazid for 2 to 3 months has resulted in cure of BCG infection. Hypersensitivity reactions may constitute a more serious problem than such infections.