Abstract Zinc protoporphyrin (ZPP) is determined by hematofluorometry of whole blood to detect iron deficiency in blood donors. In hospitalized patients, ZPP did not correlate with established markers of iron status. We performed 4500 ZPP measurements with the Aviv front-face hematofluorometer in samples from 475 patients and measured ferritin, transferrin saturation, hemoglobin, and erythrocyte indices. We found that the fluorometric determination is affected by substances dissolved in plasma but that this interference can be eliminated by using washed erythrocytes. In validation tests the within-day variation was < 3.5%; the day-to-day variation was < 6.8%. In 130 healthy persons without iron deficiency, ZPP was < or = 40 mumol/mol heme, which we consider a normal value. Mean ZPP in 46 iron-deficient patients was 256 (SD 105) mumol/mol heme (correlation with ferritin: -0.73; with hemoglobin: -0.85; P < 0.001). When washed erythrocytes are used, the hematofluorometric determination of ZPP is sensitive and specific for detecting iron deficiency in otherwise healthy individuals and hospitalized patients.