Out of 24 patients with suspected moderate to severe infections and treated with the semisynthetic aminoglycoside netilmicin, 11 patients had decreased renal function. Non-bacterial aetiology was present in five cases. There was good netilmicin effect in 10 cases, indeterminate effect in 6 cases (due to simultaneously given other antibiotics) and failure in 3 cases. Pharmacokinetic parameters were studied. Curves are given for serum concentration patterns relative to creatinine clearance, and serum half-lives are calculated in renal dysfunction. In this series of very ill patients including impaired renal function, metilmicin proved to be an efficient antibiotic with low toxicity. Only one transient minor vestibular abnormality in an 80-year-old man with concomitant diuretic medication was registered.