Twenty-one patients who received intravenous colistimethate sodium (CMS) for at least 7 days for the treatment of multidrug-resistant Gram-negative bacterial infections were included in a prospective cohort study at 'Henry Dunant' Hospital in Athens, Greece. The mean (+/- standard deviation) and median daily doses, cumulative doses and duration of treatment of intravenous CMS were, respectively, 5.5 (+/- 1.9) and 6 million IU, 90.2 (+/- 52.0) and 72 million IU, and 17.7 (+/- 11.7) and 15 days (range 7-54 days). Three patients (14.3%) developed nephrotoxicity during treatment with CMS. The cumulative dose of administered CMS was statistically correlated with the difference in values of serum creatinine between the end and start of CMS treatment (r = 0.6, P = 0.004 by Spearman's test).