At Cook County Hospital (Chicago), before 1977, the incidence of endocarditis caused by Pseudomonas aeruginosa was one or two cases per year. The frequency of P. aeruginosa as an etiologic agent of endocarditis among drug abusers increased steadily from five (23%) of 22 patients in 1977 to 15 (68%) of 22 in 1980. P. aeruginosa serotype O11 accounted for 34 (76%) of 42 of the strains serotyped. The total increase in incidence of P. aeruginosa endocarditis that we observed can be attributed to disease caused by serotype O11. We serotyped 152 strains of P. aeruginosa obtained from hospital inpatients without endocarditis. Serotype O11 was the most frequently isolated type, accounting for 27% of the total. Incidence of serotype O11 in drug addicts with endocarditis is significantly higher than the incidence in patients with nonendocarditis infections (chi 2 = 32.89; P less than .001). There was a high degree of correlation between pentazocine and tripelennamine ("T's and Blues") abuse and endocarditis caused by P. aeruginosa (chi 2 = 36.71; P less than .001).