Hematological changes were studied in physically fit young soldiers who marched 35 km/d for 6 d, working at 35% of their Vo2 max. Four days of marching produced decreases in numbers of erythrocytes (RBC) and in hematocrit (Hct). This "sports anemia" persisted beyond day 6 into the post-march period and was accompanied by decreases in hemoglobin (Hb), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), and mean corpuscular volume (MCV). The latter decrease was attributed to a preferential destruction of large RBC. The post-march period was characterized by an early (2 d) recovery of RBC numbers, Hct, and MCV, and a persistent (greater than 4 d) decrease in Hb, MCH, and MCHC. This pattern, characteristic of hypochromic macrocystosis, possibly reflects a premature release of young RBC from the bone marrow. Clearly, "Sports anemia," previously reported to occur with intensive physical exercise, can also result from sustained and repetitive submaximal exercise.