Physical training at high altitude improves performance at high altitude. However, studies assessing performance improvements at sea level after training at higher altitudes have produced ambiguous and inconclusive results. Hypoxia-induced secondary polycythemia is a major contributor to increased work capacity at altitude. The common finding upon exposure to hypoxia is a transient increase in haemoglobin concentration and haematocrit because of a rapid decrease in plasma volume followed by an increase in erythropoiesis per se. Both nonathletes and elite endurance athletes have maximal reticulocytosis after about 8 to 10 days at moderate altitude. Training periods of 3 weeks at moderate altitudes result in individual increase of haemoglobin concentration of about 1 to 4%. A more accentuated increase in haemoglobin can be obtained with longer sojourns at moderate altitude. The normal erythropoietin reaction upon exposure to hypoxia comprises initially increased levels followed by a decrease after about 1 week. Thus, the maintenance of a high erythropoietin concentration is not a prerequisite for a sustained increase in erythrocyte formation at high altitude. The main pharmacological modulator of erythropoietin production seems to be adenosine. But modulators such as growth hormone and catecholamines may also potentiate the effect of hypoxia per se on erythropoietin production. On the other hand, there is a risk that the stress hormones may induce a relative depression of the bone marrow particularly in the early phase of altitude training when the adaptation is minimal and the stress reaction is most accentuated. The most important 'erythropoiesis-specific' nutrition factor is iron availability which can modulate erythropoiesis over a wide range in humans. Adequate iron stores are a necessity for haematological adaptation to hypoxia. However, at moderate altitude, there is a need for rapid mobilisation of iron and even if the stores are normal there is a risk that they cannot be mobilised fast enough for an optimal synthesis of haemoglobin. Data from healthy athletes training at moderate altitudes suggest a true increase in haemoglobin concentration of about 1% per week. Complete haematological adaptation occurred when sea level residents have similar haemoglobin concentrations at moderate altitude compared with residents. The normal difference in haemoglobin concentrations can be estimated to be about 12% between permanent residents at sea level and at 2500m above sea level. This difference indicates a necessary adaptation time of about 12 weeks. If the training period at moderate altitude must be shorter, several sojourns at short intervals are recommended. The important factor in haematological adaptation in athletes at moderate altitude is hypoxia.(ABSTRACT TRUNCATED AT 400 WORDS)