Between 1976 and 1982, Enterobacteriaceae and Pseudomonas aeruginosa were prospectively counted in fecal specimens from leukemic patients with gram-negative bacteremia. The strains isolated from the blood and feces of 55 patients were compared. Translocation of the dominant fecal strain of Enterobacteriaceae or P. aeruginosa was observed in 45 cases (82%) and was strongly associated with granulocytopenia of less than 10(2) cells/microliter (P less than .0001). Thirteen (81%) of 16 patients with bacteremia caused by P. aeruginosa were intestinal carriers of the same strain, whereas only 2 (5%) of 39 patients with bacteremia caused by Enterobacteriaceae were carriers of P. aeruginosa. Bacterial translocation of Enterobacteriaceae was not associated with an abnormally high fecal population of the translocating strain. Prospective quantitative and qualitative analyses of fecal flora were useful in forecasting the most probable translocating gram-negative organism in neutropenic leukemic patients with clinical signs of bacteremia.