The role of reticulocytes (Ret) in sports medicine became important when the count of immature erythrocytes has been introduced in protocols used and officially approved for antidoping purposes. The use of modern automated analysers, which allow the easy count and the description of characteristics of reticulocytes, increased the possible use of these parameters in sports medicine. We studied the behaviour of Ret and immature reticulocyte fraction (IRF) in top-level athletes practising rugby, ski, soccer and cyclism, throughout a competitive season. We aimed at increasing the knowledge of physiological characteristics of these sportsmen and supplying valuable suggestions to trainers and sports physicians. We observed a stability of Ret counts, also during training and competitions, although some modifications, namely decrease during competitions periods in cyclists, and in rugby and soccer players, occurred. No significant correlation was found between Ret count and Hb in each sport discipline. IRF values tend to be high in athletes owing to continuous bone marrow stimulation linked to haemolysis, typical of sports activities. We confirm the validity of the use of Ret counts for antidoping purposes and also for evaluating health status and iron metabolism of sportsmen.