Fifty-nine consecutive patients (19 men, 40 women, mean age 60.8 [27-80] years) with primary osteoporosis were studied to see if there was any significant gain in bone mass after treatment with salmon calcitonin. All the patients were given 1 g calcium by mouth every morning. Group 1 (n = 20) received no other specific medication while group 2 (n = 19) were given 100 I.U. calcitonin subcutaneously every second evening and group 3 (n = 20) received the same dose every evening. The pain reported by the patients was subdivided into four severity grades, and analgesic consumption was recorded. In group 1 there was a nonsignificant decrease in pain, but in groups 2 and 3 there was a highly significant diminution in pain (P less than 0.005) and in analgesic intake (P less than 0.01). Measurements of bone density carried out by photon absorption at the end of 12 months showed a 5.5% increase in the distal radius in group 2 (P = 0.0001) and a 7.1% increase in group 3 (P = 0.0001), while in group 1 mineral content had decreased by 4.3% (nonsignificant). These results show that a significant gain in bone mass can be achieved by administration of calcitonin, either daily or on alternate days. The incidence of extravertebral fractures and of new or progressive vertebral deformity tended to be lower in groups 2 and 3 than in group 1.