Netilmicin (NTL), a new semisynthesized aminoglycoside, was evaluated in 11 episodes of infection in 10 patients, who had severe underlying diseases, such as acute myocardial infarction, cerebral infarction, malignancy and hepatic cirrhosis. The infection was bacteremia in 3 cases, urinary tract infections in 3 cases and respiratory tract infections in 5 cases. NTL was administered intramuscularly at a dose of 100 mg twice a day for 3 to 14 days. Overall clinical efficacy was only 40%, including excellent in 2 cases, good in 2 cases, fair in 3 cases and poor in 3 cases. Bacteriologically, 2 episodes of E. coli, 2 of S. marcescens and 1 of K. pneumoniae were eradicated, whereas, 2 of P. aeruginosa were decreased, and 1 of K. pneumoniae and 1 of P. rettgeri were persisted. Transient eosinophilia was observed in 1 case, and also nephrotoxicity was encountered in 1 case.