We evaluated the day-to-day variation of four iron-status indices: hemoglobin (Hb), hematocrit (Hct), plasma ferritin (PF), and plasma iron (PI). Finger-prick blood samples were collected for 31 consecutive days in 20 healthy men and women. Replicate (sigma 2rep) and day-to-day (sigma 2day) variance components were estimated. Day-to-day variation in the iron-status measures was similar between men and women except for PF, for which the variation was greater in women. The CVs for a single future determination for Hb, Hct, PF, and PI were 4.6%, 3.0%, 15.2%, and 26.7% for males and 4.4%, 3.2%, 26.8%, and 29.0% for females, respectively. Three to 10 independent measurements are required to accurately determine PF and PI whereas one is adequate for Hb and Hct. Thus, day-to-day biological variation is a major component of the variability in these iron-status indicators and must be considered when assessing iron status.