The main objective of the present study was to characterize the growth patterns of Lusitano horse managed on grazing systems from birth to 42 month of age. A total of 4759 records for body weight (BW), withers height (WH), girth (G) and cannon circumference (CC) were obtained from 121 Lusitano foals (62 colts and 59 fillies) born and raised at four stud-farms. Data were regularly collected between birth and 42 months of age, when foals began to be ridden. Several sigmoid growth functions were adjusted using the NLIN procedures of SAS. The Richards equation was chosen for further analysis because it was the best fit model for all the variables. Growth rates (ADG, kg/d or cm/d) were obtained from the first derivative of the equations. The influence of season was examined and the effect of gender was tested. The mean mature BW was 552.4±22.0 kg and the average mature sizes were 158.6±0.8 cm, 197.8±3.6 cm and 19.9±0.2 cm, respectively for WH, G and CC. Proportions (%) of mature BW at 6, 12, 18, 24, 30, 36 and 42 months of age, according to the Lusitano breed standard were, respectively, 42, 58, 69, 77, 84, 89 and 93%. The proportions (%) of mature WH for the same ages were 82, 88, 92, 94, 95, 96 and 97%. Concerning BW, Lusitano showed a slower growth rate, comparable with the moderate growth proposed for other sport breeds. In contrast, the WH growth rate was similar to those presented by early maturing breeds. A clear sexual dimorphism was observed on all variables, with higher maturity values for males (P<0.0001). The present study provides a comprehensive overview on growth patterns of the Lusitano horse managed in grazing systems under Mediterranean climate conditions. Further studies with controlled feeding levels should be done in order to verify the potential growth response of this breed.