Because of increased aminoglycoside resistance of hospital bacterial isolates, aminoglycoside sensitivity patterns of isolates in a large children's hospital were assessed before and during a 33-month period of almost exclusive amikacin use. There was no significant change in overall resistance rates of gram-negative enteric bacteria to gentamicin (4.8 percent and 4.6 percent), tobramycin (2.5 percent and 3.6 percent), and amikacin (1.2 percent and 1.8 percent) from the pre-amikacin period to the amikacin usage period, respectively. No significant differences were observed for isolates of Escherichia coli, Klebsiella, Serratia, Acinetobacter, and Pseudomonas species. In contrast, significant decreases in gentamicin and tobramycin resistance rates for Enterobacter, Citrobacter, and Pseudomonas aeruginosa and in gentamicin resistance of Proteus were found. Very little change in resistance of staphylococcal isolates was seen during a shorter evaluation period. Pediatric aminoglycoside usage includes therapy of neonatal infections, cystic fibrosis, febrile neutropenic episodes in patients with cancer, abdominal surgery, bacterial endocarditis, and gram-negative central nervous system infections. Amikacin has also been used successfully as single-dose therapy of urinary tract infections, and acceptable cerebrospinal fluid levels of amikacin have been documented in hydrocephalic patients with ventriculitis. In vitro studies of 22 bacterial isolates demonstrated synergy between amikacin and penicillin or newer cephalosporins in 13, an additive effect in seven and indifference in two. No antagonism was found. In addition, in vivo synergy between imipenem and amikacin was found in neutropenic infant rats with P. aeruginosa sepsis using a strain with which no synergy was demonstrable in vitro. Amikacin is effective in pediatric infections and is well tolerated by children. Because excessive or inadequate levels are frequent with usually recommended doses, particularly in neonates and patients with compromised renal function or cystic fibrosis, serum levels should be monitored to minimize risk and to ensure therapeutic levels.