The presentation discusses effectiveness of early orthodontic treatment, carried out with an activator -type appliance (eruption guidance appliance) in the early mixed dentition. Findings are based on a prospective cohort study where treatment and control groups included children with all types of malocclusion except posterior crossbite and Class III. After active treatment, 90% of children in treatment group had a full Class I relationship, small overbite and overjet and straight and well-aligned dental arches. During the same time, malocclusions in the control group stayed unchanged or got worse. The skeletal effcets of the treatment included mainly dentoalveolar changes but also a significant increase in the length of the mandible. Examination at the age of 16 to 17 years of age, after a minimum of two years out of retention, revealed that the treatment results were stable.