The purpose of the present study was to identify factors which might influence the success of autotransplantation, such as root development, root resorption, pulp condition and tooth eruption. The material consisted of 100 human premolars transplanted in 87 patients. The observation period ranged from 3-18 years, with a mean value 6.3 years. Clinical and radiographic examinations were performed. The transplanted premolars were divided into 7 stages of root development. At the final control, 93 teeth remained and 7 teeth were extracted. Periodontal healing without root resorption was related to stage of root development. It decreased from 93% in teeth with 3/4 root development to 37% in teeth with fully developed roots. Inflammatory root resorption was seen in 7 cases and replacement root resorption in 12 cases. Pulp revascularization was observed in 100% in stages with initial root development to 1/2 root development and decreased to 0% for teeth with fully developed roots. Pulp obliteration was observed in all cases with revascularized pulp tissue. The gingival condition was similar to contralateral, not transplanted premolars or adjacent teeth. Root growth increased with the stage of root development at the time of transplantation. It is concluded that transplantation of premolars with 1/2-3/4 root development provides a good chance of pulp survival, limited risk of root resorption and ensures sufficient final root length, and is thus recommended.