Objective: To determine whether birth date (month of birth) influences selection in basketball. The relative age effect (RAE) will show whether there is variation in the percentage of number of births throughout the natural year. Method. The months of birth of players of the Association of Basketball Clubs (Asociación de Clubes de Baloncesto [ACB]), the Spanish Basketball League (Liga Española de Baloncesto [LEB1, LEB2]), the National Basketball Association (NBA) and of grassroots players of several clubs were identified. Data were grouped in trimesters. Results. A strong tendency to select players born in the first trimesters of the year compared with the last trimesters was found. This tendency was maintained but was less apparent in the higher categories up to professional level. An explanation for this finding is that in the first stages of basketball, boys are selected on the basis of advanced maturation or by one of its indicators (height). Thus, many future talents tend to be lost and other boys with lower aptitude may reach professional level simply because they were born at the beginning of the year. Conclusions. Month of birth strongly influences the selection of basketball players in Spain. Appropriate selection criteria should be established to optimize the selection and development of future players.