Objective: To determine the extent of elimination of trimethoprim (TMP) and sulfamethoxazole (SMX) via continuous venovenous hemodiafiltration (CVVHDF) in 2 critically ill patients with renal failure. Case Summary: A 62-year-old woman with Pneumocystis jirovecii pneumonia (PCP) was admitted to our intensive care unit for severe acute respiratory distress syndrome. A 77-year-old man was admitted for aortic root replacement and developed septic shock and nosocomial pneumonia due to Stenotrophomonas maltophilia. Both patients developed acute renal failure, necessitating CVVHDF. They were treated with intravenous TMP/SMX adapted to renal function. The first patient received TMP 6.4 mg/kg/day and SMX 32 mg/kg/day, corresponding to 50% of the recommended high-dose TMP/SMX regimen in PCP patients. The second patient received TMP 1.7 mg/kg/day and SMX 8.6 mg/kg/day, corresponding to 50% of the usual dose in bacterial infections. We determined peak and trough serum TMP and SMX concentrations and the extent of TMP/SMX CVVHDF clearance at steady-state while the patients were still anuric and oliguric. Discussion: Data on TMP and SMX pharmacokinetics in CVVHDF are lacking and dosing recommendations are inconclusive. In both patients, CVVHDF clearance of TMP ranged from 21.5 to 28.9 mL/min, corresponding with normal renal clearance (20–80 mL/min). SMX clearance in CVVHDF showed high variability (18.7, 26.7, and 42.6 mL/min) and exceeded renal clearance values in normal renal function (1–5 mL/min). Accordingly, peak TMP serum concentrations were within the recommended range in the patient treated with a reduced TMP/SMX dose for PCP, whereas her SMX peak concentrations were only one third of recommended target concentrations. Conclusions: Our data indicate that both TMP and SMX are removed by CVVHDF to a significant degree, and dose reduction of TMP/SMX in CVVHDF bears the risk of underdosing. Given variability in drug exposure in critically ill patients, therapeutic drug monitoring is advisable in anuric or oliguric patients undergoing continuous renal replacement therapy to ensure optimal TMP/SMX dosing.