The effect of unilateral extraction of the first or second primary molar on space conditions, need for orthodontic treatment, and eruption stage of permanent canines and second molars has been studied longitudinally in two groups. In Group 1 the first primary molar had been extracted early in 13 cases in the maxilla and in 14 cases in the mandible. In Group 2 the corresponding figures for the second primary molar were 14 and 24 for the maxilla and mandible, respectively. The space on the extraction side in Group 1 was significantly less than on the control side at the age of 9 but not at the age of 13. In Group 2 there was significantly less space on the extraction side than on the control side at the two registration ages (10.5 and 12 years). Need for orthodontic treatment was low in Group 1 and high in Group 2. As for eruption stage of permanent canines and second molars, no significant differences were found between the extraction side and the control side in the two groups.