This retrospective case-control study compared the tolerability and efficacy of linezolid between patients with and without renal insufficiency (serum creatinine > or =1.3mg/dL for women and > or =1.5mg/dL for men). All patients with Gram-positive infections treated with linezolid for more than 7 days were included. Data were collected from medical charts and differences between patients with and without renal insufficiency were analysed. Sixty-two patients (40 men), with a mean age of 56.9 years, were enrolled in this study. At the start of linezolid treatment, 17 patients (27.4%) had impaired renal function. Patients with renal insufficiency had a higher prevalence of diabetes mellitus compared with those with normal renal function (64.7% vs. 22.2%; P=0.002). At the start of therapy, patients with renal impairment had a higher frequency of elevated blood urea nitrogen (51.0+/-21.1mg/dL vs. 18.3+/-9.7mg/dL; P<0.001), elevated serum creatinine (2.3+/-0.7mg/dL vs. 0.9+/-0.3mg/dL; P<0.001) and decreased initial haemoglobin (9.2+/-1.5g/dL vs. 10.4+/-1.7g/dL; P=0.017). Development of severe thrombocytopenia (<100 x 10(9)/L) was significantly more common in patients with renal insufficiency (64.7% vs. 35.6%; P=0.039). The incidence of linezolid-associated thrombocytopenia was higher among patients with renal insufficiency. When patients with renal insufficiency are treated with linezolid for more than 2 weeks, the platelet count should be monitored at least twice a week owing to the increased likelihood of thrombocytopenia.