Abstract The purpose of this study was to compare the died of a monthly refilling of the root canals with calcium hydroxide paste with a single packing or replacement of the paste at 3 months on I he apexification of nonvital maxillary incisors of vervet monkeys. Forty-eight maxillary incisors from 12 monkeys were used following radiographic determination that root development was incomplete. The pulps were extirpated under general anesthesia and the root canals filed and cleaned. The root canals were filled with a commercial calcium hydroxide paste, Calxyl, and a temporary cavity filling placed. Twelve teeth were left without further treatment. The calcium hydroxide paste was replaced in 12 teeth alter a 3-month interval, and in the remaining 24 teeth the calcium hydroxide root filling was replaced live times at monthly intervals. The monkeys were killed after 6 months, and blocks of the teeth and surrounding tissues were embedded, decalcified and 6 μm serial sections prepared and stained. The sections were studied histologically to evaluate 11 parameters. Significant differences were found in the amount of calcium hydroxide at the apices, the presence of new cementum on the roots and the degree of inflammation, all of which were better in the monthly refill group. Histomorphometric measurements to evaluate the obturation of the open apices and the volume of new primary osteocementum showed no significant difference between the three groups. It was suggested that after the initial root filling with calcium hydroxide there was nothing to be gained by repeated root filling either monthly or after 3 months, for al least 6 months.