To determine the association of aminoglycoside plasma levels with therapeutic outcome in gram-negative pneumonia, the case reports of 37 patients from four prospective, randomized, controlled trials of gentamicin, tobramycin, and amikacin were analyzed. Twenty (54 percent) of these patients had a favorable outcome. Patients with maximal one-hour postinfusion (peak) levels of 7 micrograms/ml or greater for gentamicin and tobramycin or 28 micrograms/ml or greater for amikacin more often had successful outcomes (14 of 20, 70 percent) than those with levels less than this (six of 19, 32 percent) (p less than 0.006). Patients with overall mean peak levels of 6 micrograms/ml or greater for gentamicin and tobramycin or 24 micrograms/ml or greater for amikacin more often had successful outcomes than those with levels less than this (six of 17, 35 percent) (p less than 0.04). The initial patient temperature, serum urea nitrogen/creatinine ratio, initial polymorphonuclear leukocyte count, and age were also associated with outcome; but by multivariate analysis, achieving an adequate peak concentration was the most important discriminating factor. These results suggest the potential importance of achieving adequate aminoglycoside levels in patients with gram-negative pneumonia.