Hormonal, immunological, and hematological responses to intensified training in elite swimmers. Med. Sci. Sports Exerc., Vol. 29, No. 12, pp. 1637-1645, 1997. The purpose of this study was to compare the responses of selected hormonal, immunological, and hematological variables in athletes showing symptoms of overreaching with these variables in well-trained athletes during intensified training. Training volume was progressively increased over 4 wk in 24 elite swimmers (8 male, 16 female); symptoms of overreaching were identified in eight swimmers based on decrements in swim performance, persistent high ratings of fatigue, and comments in log books indicating poor adaptation to the increased training. Urinary excretion of norepinephrine was significantly lower (P < 0.05, post hoc analysis) in overreached (OR) compared with well-trained (WT) swimmers throughout the 4 wk. There were no significant differences between OR and WT swimmers for other variables including: concentrations of plasma norepinephrine, cortisol, and testosterone, and the testosterone/cortisol ratio; peripheral blood leukocyte and differential counts, neutrophil/lymphocyte ratio, and CD4/CD8 cell ratio; serum ferritin and blood hemoglobin concentrations, erythrocyte number, hematocrit, and mean red cell volume (MCV). MCV increased significantly over the 4 wk in both groups, suggesting increased red blood cell turnover. These data show that, of the 16 hormonal, immunological, and hematological variables measured, urinary norepinephrine excretion appears to be the only one to distinguish OR from WT swimmers during short-term intensified training. Low urinary norepinephrine excretion was observed 2 to 4 wk before the appearance of symptoms of overreaching, suggesting the possibility that neuroendocrine changes may precede, and possibly contribute to, development of the overreaching/overtraining syndromes.