Forty patients had staphylococcal endocarditis and a history of parenteral drug abuse. Clinical and microbiologic features of their cases were evaluated. None of our patients were known to have had preexisting valvular disease. The tricuspid valve lesions and their pulmonary complications were the predominant findings. Systemic complications in the form of meningitis, glomerulonephritis, empyema, arthritis, and nosocomial Gram-negative septicemia occurred in 33% of our patients. Of interest was the high incidence of reactions to therapy especially with methicillin sodium, which occurred in 30% of patients. Correlation of phage type and group with the antibiotic sensitivities of individual staphylococci showed that group 3 and phage types 6, 42E, 54, and 75 were much more resistant to penicillin than other groups and types. The clinical outcome did not relate to phage type and group or to antibiotic sensitivity of the organism.