Introduction: Immature red blood cell parameters compose of the anti-doping programs of several sports, including soccer. The biological variability of reticulocytes is not well defined in young athletes. Our aim was to calculate biological variation and reference change values (RCV) for immature red blood cell parameters in young soccer players.Methods: Samples from 19 male soccer players (mean age, 18 ± 1 years) were collected before the start of training (C0) and after two (C1) and four weeks of training. Blood samples were collected in tubes with K3-EDTA Vaccuete® (Greiner Bio-One). Red blood cell parameters were analyzed on Sysmex XE-5000®. The e-Check Sysmex® 2 levels were used to obtain the coefficient of analytical variation (CVA). Homogeneity of variance was verified using the Cochran test. The within-subject (CVI) and between-subject biological variation (CVG) was calculated according to the mean and standard deviation from the athletes’ results. ANOVA with repeated measures was used to compare the means at the significance level of p<0.05. RCV95% was calculated using the Fraser’s formula: RCV=21/2 x1.96x(CVA2 + CVI2)1/2. Graph Pad Prism 6.0 and Matlab 7.0 were used to perform statistical analyzes.Results: Mean corpuscular volume, RDW, and reticulocyte hemoglobin content (Ret-He) had significant increases during the training period. The Ret-He were higher in C1 (32.4±1.1 pg) and C2 (32.3±1.3 pg) compared to C0 (31.9±1.2 pg) (p<0.05). It was not possible to calculate RCV for Ret-He because of heterogeneous variances. The RCV of reticulocytes (29.1%) and IRF (48.8%) were higher than Hgb (5.5%) and Hct (6.7%).Conclusion: Reticulocytes are highly variable in different athletes’ and the RCV obtained from young soccer players would contribute to monitor training adaptation and to diagnose sports anemia. The Ret-He behavior in our study suggests that this parameter can detect an early increase of iron for hemoglobin synthesis in response to training, this makes it an interesting biomarker to assess anemia and suspicion of doping by rhEPO.