Acinetobacter calcoaceticus var anitratus is an increasing nosocomial problem in some institutions. Duirng a recent 16-month period, we observed 15 patients with blood isolates of this organism. Thirteen of these patients acquired the organism from the hospital environment. Eleven of these represented real disease and concurrent growth of the organism from another site was predictive of infection (P less than 0.03). A review of laboratory records showed that there was a three-fold increase in nonblood isolates of this organism from hospitalized patients in 1975 and 1976, as compared to 1972 (P less than 0.0001), and a marked seasonal effect was noted, with increased isolations during the summer. Aminoglycoside resistance had increased considerably with 25% of nonblood isolates being resistant to gentamicin and 16% resistant to tobramycin despite its restriction; blood isolates were uniformly susceptible to gentamicin and tobramycin. Reporting of A anitratus as a nosocomial pathogen and serotyping of isolates may be useful to further define its role in nosocomial infections.