The aim of this study was to investigate the clinical and economic significance of aminoglycoside peak concentrations in febrile neutropenic patients with hematologic malignancies. Sixty-one patients were treated according to protocol II of the Paul-Ehrlich-Gesellschaft: initial application of gentamicin or tobramycin in combination with a cephalosporin or ureidopenicillin and, after 3 days, a potential change of antibiosis to be decided in case of nonresponse. At the same time, samples were collected by an independent controller. We found a significant dependence of clinical outcome on aminoglycoside peak concentrations (P = 0.004). Twelve of 17 patients with peak concentrations > 4.8 mg/L, but only 13 of 44 patients with concentrations < or = 4.8 mg/L, responded to initial therapy. Average infection-related costs per patient with peak values > 4.8 mg/L were US$1429, $1790, and $1701 for nursing, diagnostics, and therapeutics, respectively (total $4920). Expenses for patients with peak concentrations < or = 4.8 mg/L were approximately 1.8-fold higher (average total $8718). If all 61 patients had achieved peaks > 4.8 mg/L, the potential savings would have totalled $167,112. We conclude that neutropenic patients form a target group for successful pharmacokinetic intervention and cost saving.