Consecutive patients with serious infections were randomized between gentamicin 4 mg/kg once daily i.v. or netilmicin 5.5 mg/kg once daily i.v. (with dosage reduction in case of renal dysfunction). Exclusion criteria were neutropenia or severe renal failure. Median first serum trough and peak concentrations were 0.4/9.5 mg/L and 0.4/12.2 mg/L, for gentamicin and netilmicin respectively. A good clinical response was observed in 50/54 (92.6%) evaluable patients treated with gentamicin and in 48/52 (92.3%) netilmicin-treated patients. Nephrotoxicity (a rise of serum creatinine > or = 45 mumol/L) developed in 5/72 (6.9%) gentamicin patients treated > or = 48 hours and in 10/69 (14.5%) netilmicin patients (difference 7.5%, 95% CI -3.9% to +16.2%). High-tone audiometry was performed when possible; no significant differences were found between the regimens with regard to hearing loss or prodromal signs of ototoxicity. We conclude that with once-daily dosing no benefit of netilmicin over gentamicin regarding nephro- or ototoxicity could be demonstrated.