Hematologic parameters and serum ferritin levels were measured in groups of experienced ultra-marathon runners under control conditions, 2 days after a 160-km ultra-marathon, and for up to 14 days after a 56-km ultra-marathon. Under resting conditions, 14% of the runners had subnormal serum ferritin levels compared to 2% of a control group and not one of a group of trained swimmers. Serum ferritin levels that were markedly elevated after both ultra-marathon races returned to pre-race levels only 6 days after the 56-km ultra-marathon and continued to fall in athletes who did not exercise for a further 8 days. Other hematologic changes that were present after either of the ultra-marathon races included: immediate post-race hemoconcentration (shown by increased mean red cell count, hemoglobin level, and packed cell volume) and increased mean corpuscular volume, followed by hemodilution that was greatest 48 h after the 160-km race; an increased mean corpuscular hemoglobin concentration and reticulocyte production index; transient leukocytosis, monocytosis, lymphocytopenia, eosinophilopenia, and the appearance of band cells. With the exception of the increase reticulocyte production index and the reduced packed cell volume, all other hematologic parameters had returned to control levels 6 days after the 56-km race. This study shows that serum ferritin levels may be subnormal in a proportion of distance runners and that daily training and ultra-marathon racing in particular may cause these levels to remain elevated for between 6--14 days. Thus, when hematologic parameters are measured in distance runners, it should be remembered that recent prolonged exercise may (1) produce a "dilutional anemia," (2) by increasing serum ferritin levels, mask a true iron deficiency, and (3) that these changes may require up to 6 days to return to normal.