To determine the prevalence and time course of thiamine deficiency (TD) in PICU patients.Multicenter, prospective, cohort study between May 2019 and November 2019.Three university-based tertiary care, mixed medical-surgical PICUs in Ankara, Turkey.PICU patients 1 month to 18 years old.None.We studied 476 patients and grouped them by TD status on days 1 and 3 of the PICU admission. There might be a risk of unintended bias since we excluded 386 patients because of the absence of consent, inadequate blood samples, loss of identifier information, and recent vitamin supplementation. On day 1, TD was present in 53 of 476 patients (11.1%) and median (minimum-maximum) thiamine levels were 65.5 ng/mL (5-431 ng/mL). On day 3, TD was present in 27 of 199 patients (13.6%) with repeated measurement. The median (minimum-maximum) thiamine levels were 63 ng/mL (13-357 ng/mL). The time course of TD from day 1 to day 3 in these 199 patients was as follows. In 21 of 199 patients (10.6%) with TD on day 1, 11 of 21 (52%) continued to have TD on day 3 and the other 10 of 21 patients (48%) improved to no longer having TD. In 178 of 199 patients (89.4%) without TD on day 1, 16 of 178 (9%) went on to develop TD by day 3, and the other 162 of 178 (91%) continued to have normal thiamine status.In the PICU population in three centers in Turkey, the prevalence of TD in the sample of patients was 11.1%. In those TD patients who had serial studies, we also identified that by day 3 some continued to be TD, and some patients improved to normal thiamine status. Of concern, however, is the population who develop TD over the course of PICU stay.