Background The aim of the study was to determine serum thiamin concentrations in critically ill medical patients who required chronic diuretic drug treatment before admission to a medical intensive care unit (ICU). Methods This prospective study was performed in a medical ICU. Subjects who received diuretic drug therapy for at least 6 months prior to ICU admission constituted the diuretic group. The control group was clinically matched adults admitted to the same ICU but without a history of diuretic therapy. Results A total of 50 subjects were included (25 subjects in each of the diuretic and control groups). In the diuretic group, daily dose of furosemide prior to admission was 40 mg/d (range of 20-160 mg/d). In all subjects, the ICU admission baseline blood thiamin concentrations were 31.2 +/- 27.1 ng/mL. In the diuretic group, the baseline whole blood thiamin level was significantly lower compared with levels in the control group (15.5 +/- 10.7 vs 46.8 +/- 29.5 ng/mL; P < 0.001). On day 2 after entry, thiamin levels remained low (23.2 +/- 15.4 ng/mL in the diuretic group vs 49 +/- 38 ng/mL in the control group; P = 0.003). Low thiamin levels were found in 96% of patients at baseline and in 72% of patients on the second day in the diuretic group. Conclusion Adults receiving chronic diuretic therapy and then requiring medical ICU care commonly exhibit thiamin depletion on admission to the ICU and during the initial days of ICU care.