In intervention and observational studies, it is necessary to determine the number of blood samples required to estimate the true value of Fe-status measures. The aim of the present study was to determine the number of days for blood sampling required in order to measure the 'true value' of five Fe-status parameters in young Fe-depleted women and to investigate the effect of menstrual cycle on these measures. Twelve women (aged 23-30 years), non-anaemic but with low Fe stores, participated in the study. Venous blood samples were collected under standardised conditions on fifteen non-consecutive days during a 5-week period. All blood samples were analysed for Hb, serum ferritin (SF), serum transferrin receptors (sTfR), red blood cell volume distribution width (RDW) and reticulocytes (RET), and body Fe stores were calculated as the ratio between sTfR and SF. No systematic changes were found in the investigated parameters during the study. When analytical variations were accounted for, the day-to-day variations (CV%) were as follows: Hb 2.9 %, SF 8.2 %, RET 26.0 %, RDW 2.4 % and sTfR 8.1 %. Calculating the 'true value' with a 5 % significance level and 80 % power showed that one blood sample was sufficient for Hb, SF, sTfR and RDW, whereas seven blood-sampling days were needed for RET. In this study, no significant differences in Fe status were found across the menstrual cycle. The conclusions from this study are valid for studies conducted under similar strict conditions.