Background: Alterations in hematology, especially erythroid changes, may be involved in acute mountain sickness(AMS) at high altitude. This study aimed to identify the relationship between excessive erythrocytosis and AMS following different durations of high-altitude exposure.Methods: A total of 692 healthy young Chinese men were recruited for the study in June and July of 2012 and were divided into the following five groups: I) the 24-h group(24 hours after arrival at Lhasa, 3,700 m, n=261); II) the 7-d group(exposed at Lhasa, 3,700 m for seven days, n=99); III) the re-exposure group(re-exposed at Yang Bajing, 4,400 m for seven days after >1 year of acclimation at 3,700 m, n=94); IV) the acclimated group(>1 year of acclimation at 3,700 m, Lhasa, n=42); and V) the sea-level control(control group, Chengdu, n=196). Case report forms were used to record the subjects’ demographic information and AMS-related symptoms. All of the subjects underwent routine blood tests.Results: The red blood cell(RBC) count fell slightly but was not significant upon acute exposure to high altitude, whereas the hemoglobin concentration([Hb]) increased significantly. After high-altitude re-exposure, both of the [Hb] and RBC count showed significant increases. The incidence of AMS was 65.1%, 26.3% and 51.1%, respectively in the 24-h, 7-d and re-exposure groups. The [Hb](P=0.024) and hematocrit(P=0.017) were greater in the AMS+ individuals than in the AMS– individuals in 7-d group. A correlation analysis revealed that the [Hb] and hematocrit were closely related with AMS score in 7-d and re-exposure groups, while the RBC showed a correlation with AMS score only in the re-exposure group. The AMS incidence was lowest when the [Hb] was between 140 and 160 g/L in the 24-h and 7-d groups.Conclusions: AMS is associated with both [Hb] and excessive erythrocytosis. Additionally, our findings indicate the existence of an optimal [Hb] for preventing AMS. Background: Alterations in hematology, especially erythroid changes, may be involved in acute mountain sickness (AMS) at high altitude. This study aimed to identify the relationship between excessive erythrocytosis and AMS following different durations of high-altitude exposure. Methods: A total of 692 healthy young men were recruited for the study in June and July of 2012 and were divided into the following five groups: I) the 24-h group (24 hours after arrival at Lhasa, 3,700 m, n = 261); II) the re-exposure group (re-exposed at Yang Bajing, 4,400 m for seven days after> 1 year of acclimation at 3,700 m , n = 94); IV) the acclimated group (> 1 year of acclimation at 3,700 m, Lhasa, n = 42); and V) the sea-level control were used to record the subjects’ demographic information and AMS-related symptoms. All of the subjects underwent routine blood tests. Results: The red blood After high-altitude re-exposure, both of the [Hb] and RBC count were significantly significant The incidence of AMS was 65.1%, 26.3% and 51.1% respectively in the 24-h, 7-d and re-exposure groups. The [Hb] (P = 0.024) and hematocrit the AMS + individuals than in the AMS-individuals in 7-d group. A correlation analysis revealed that the [Hb] and hematocrit were closely related with AMS score in 7-d and re-exposure groups, while the RBC showed a correlation with AMS The AMS incidence was lowest when the [Hb] was between 140 and 160 g / L in the 24-h and 7-d groups. Conclusions: AMS is associated with both [Hb] and excessive erythrocytosis. Additionally, our findings indicate the existence of an optimal [Hb] for preventing AMS.