Long term diuretic therapy represents one central pharmacologic therapy of heart insufficiency and hypertension. Diuretics lead not only to an increased urinary excretion of electrolytes but also of water soluble vitamins. In this prospective study we evaluated the effect of hospitalization on the overall biochemical vitamin status in subjects older than 50 years (n=149, mean +/- SD age 70 +/- 10 years). Vitamin nutriture and other parameters were assessed at admission and discharge (duration of the hospitalization 19 +/- 1 day). Only vitamin B1 nutriture worsened during the hospitalization and in a multivariate procedure the only significant predictor of the change in the vitamin B1 nutriture was the use of diuretics during the hospitalization (F=4.06, p < 0.001). The changes in the ETK (erythrocyte transketolase activity in whole blood) and a-ETK (ETK activity coefficient) during the hospital stay correlated with the cumulative dosage of furosemide adjusted for the duration of the therapy (r = 0.36, p < 0.001 and r = - 0.28, p > 0.03). Our data suggest that hospitalized elderly are at increased risk for vitamin B1 deficiency especially when on a diuretic treatment. It is possible that a low dose thiamine supplementation my help to prevent the development of a subclinical wet-beriberi in older subjects on diuretics.