The present invention relates to a modified-release composition of mazindol and its use in the treatment of attention deficit disorders (ADD) or attention deficit/hyperactivity disorder (ADHD) or related deficit of alertness (i.e., incoercible sleepiness) or decline of vigilance (i.e., daytime somnolence) or excessive daytime sleepiness (e.g., narcolepsy, idiopathic hypersomnia) in particular in children, adolescents and adults.