A computerized system for testing and surveillance of bacterial susceptibility to antibiotics was used in monitoring the emergence of gentamicin-resistant strains of aerobic and facultative gram-negative bacilli at Massachusetts General Hospital since the release of gentamicin for clinical use in 1971. During the period studied, there was a significant increase in the prevalence of gentamicin-resistant bacteria, particularly among Pseudomonas, Acinetobacter (Herellea), and Proteus and, more recently, among Enterobacter and Klebsiella. Most gentamicin-resistant strains of Pseudomonas aeruginosa and Acinetobacter calcoaceticus var. anitratum (Herellea varginicola) retained susceptibility to tobramycin. Of the other gentamicin-resistant organisms, most were also resistant to tobramycin. Twelve patients with infections caused by gentamicin-resistant organisms were treated with tobramycin. All 12 patients were seriously ill, and all but one had failed to respond to previous therapy with gentamicin. Nine patients responded favorably to tobramycin, and six were cured. P. aeruginosa and A. calcoaceticus var. anitratum were most frequently the infecting organisms in these patients. Patients received tobramycin for three to 42 days; no significant drug-related toxicity was noted. These results emphasize the increasing clinical importance of gentamicin-resistant bacteria and suggest that tobramycin may be effective for treatment of some, but not all, infections caused by gentamicin-resistant bacteria.