The present study was designed to define the hemoglobin [Hb] increase with altitude in Peruvian children. We suggest the normal range of [Hb] as means +/-2 standard deviations (SD), with a value less than - 2 SD as a possible threshold to detect anemia. The prevalence of anemia was calculated. These values were compared to the World Health Organization (WHO) altitude correction parameter and the threshold for anemia of 11 g/dL. Likewise, polycythemia is suggested as [Hb] greater than 2 SD. 2,028,701 children aged 6-59 months were analyzed. The quadratic regression analysis shows that [Hb] is constant between sea level and 999 m. Thereafter, [Hb] increases from 11.32 g/dL (1000 m) up to approximately 14.54 g/dL at 4000 m. Applying the threshold for anemia defined by WHO (11 g/dL) results in a prevalence of approximately 35% for children living at altitudes <1000 m, and prevalence decreases to approximately 4.5% at >4000 m. After [Hb] altitude correction, the prevalence was approximately 36% (1000 m) and increases to approximately 66% above 4000 m. With our proposed threshold for anemia, the prevalence was approximately 15% below 1000 m and approximately 5% above 4000 m. For polycythemia ([Hb] >14.5 g/dL), increases were from 1.2% at <1000 m to 39.4% at 4000 m. After [Hb] correction for altitude, the prevalence of polycythemia decreases with altitude. Excessive erythrocytosis defined as [Hb] >19 g/dL shows the highest values at 4000 m, while polycythemia defined as [Hb] greater than 2 SD was reduced at high altitude (HA). In conclusion, using WHO thresholds for anemia and [Hb] correction by altitude most likely overestimates the prevalence of anemia and may underestimate polycythemia in Peruvian children living at HA. Therefore, new threshold values for anemia and polycythemia as mean [Hb] less than 2 SD and greater than 2 SD for populations living at a specific altitude are suggested.