OBJECTIVE To describe the pharmacokinetics of flucytosine in a critically ill patient undergoing continuous venovenous hemodiafiltration (CVVHDF) treated for cryptococcal meningitis. CONCLUSIONS A 20-year-old female weighing 93.4 kg with a body mass index of 34.3 kg/m2 with a past medical history of systemic lupus erythematous with diffuse proliferative lupus nephritis (class IV) was admitted to the hospital after several months of worsening dyspnea, fatigue, myalgia, vomiting, and diarrhea. The patient developed worsening renal function and volume overload requiring CVVHDF on hospital day 7. She was diagnosed with cryptococcal meningitis on hospital day 8, and flucytosine 2,500 mg enterally every 12 hours and liposomal amphotericin B 500 mg intravenously every 24 hours were initiated. Flucytosine serum concentrations were collected on day 4 of therapy, and pharmacokinetics were performed on 2 sequential levels. Pharmacokinetic calculations displayed an elimination rate constant of 0.0338 h-1, a volume of distribution between 0.42 and 0.43 L/kg, a half-life of 20.5 hours, and a total drug clearance between 1.32 and 1.36 L/h while on CVVHDF. The nonsequential levels displayed good correlation, and no further monitoring or dosage adjustment was required. The patient completed therapy, with clinical resolution of her infection, and no toxicities due to flucytosine were noted. CONCLUSIONS Flucytosine dosed at 25 mg/kg of actual body weight every 12 hours during CVVHDF conferred therapeutic levels with no appreciable toxicities. Because of its narrow therapeutic index and risk of toxicity, additional pharmacokinetic studies are needed to determine optimal drug dosing of this medication in patients requiring renal replacement therapy.