The correlation of fecal gram-negative bacilli (GNB), neutropenia, and bacteremia was studied in 45 bone marrow transplant recipients. Weekly stool cultures were prospectively monitored for GNB resistant to routine prophylactic and empiric antimicrobial agents. Seven cases of GNB bacteremia occurred in 45 patients described as follows. Twenty-three patients had no fecal or blood GNB. Fifteen patients had fecal GNB and no blood GNB; three of these latter patients had less than or equal to 50/mm3 circulating white blood cells (WBC) at the time of isolation of fecal GNB but two of the three were concurrently receiving appropriate empiric antibiotics. Two patients had blood GNB but no fecal GNB: one patient had a trimethoprim/sulfamethoxazole (TMP-SMZ)-sensitive isolate that would not be detectable in the feces by our methodology and one patient had feces analyzed only after the bacteremic event. Five patients had fecal GNB and blood GNB: one of these patients did not have a fecal sample analyzed prior to bacteremia but the remaining four patients had the same species/antibiogram of GNB isolated from the feces two to three days prior to the detection of bacteremia. Thus, the fecal GNB could have been used to predict the antibiogram of the subsequent blood GNB. In addition, all four of these latter bacteremic patients had less than or equal to 50/mm3 circulating WBC at the time of documented fecal GNB. Thus, bone marrow transplant recipients with fecal GNB coupled with severe neutropenia (less than or equal to 50/mm3 circulating WBC) were more likely to develop bacteremia (P less than 0.02) than were those with fecal GNB and greater than 50/mm3 circulating WBC.