Strict regulations exist for the selection of blood donors. These include minimum hemoglobin (Hb) standards of 13.5 and 12.5 g/dL for males and females respectively. In Canada 2% of all blood donors, or approximately 25,000 individuals annually attempt to make a blood donation but are deferred because their Hb level does not meet these minimum accepted standards. In a previous study we provided a biostatistical approach to ascertain objectively the optimal minimum Hb standards for blood donors to best discriminate between iron deficient and non iron deficient individuals. The derived values were lower than the existing minimum Hb standards and because of concern that blood donors accepted using these lower Hb standards might have asymptomatic disease, the proposed new levels were not adopted. This present prospective study was undertaken to assess the impact on the blood donors of the new Hb standards. Over 26 months we evaluated 1,558 donors (695 males and 863 females). On entry into the study they were screened for a variety of medical conditions which could result in asymptomatic anemia and 6 months later they answered a health questionnaire. Blood donors were entered into 1 of 3 groups. Group I were donors accepted by the existing criteria. Group II were donors who did not meet the existing Hb criteria but satisfied the newly derived standards. Group III were donors deferred even by the new Hb standards.(ABSTRACT TRUNCATED AT 250 WORDS)