Since data on the efficacy of 4-amino-2-hydroxybutylidene-1,1-bisphosphonate (AHBuBP) in the therapy of osteoporosis are not yet available, we have examined in an open, randomized study, the effects of an intravenous intermittent treatment with this drug on the vertebral and radial bone mass in postmenopausal osteoporosis. Forty postmenopausal osteoporotic women were randomly assigned to one of two groups, one treated with AHBuBP (5 mg/day on two consecutive days every 3 months for one year; n = 20), the other with oral calcium (n = 20). The bone mineral density (BMD) of the spine increased significantly in women receiving AHBuBP, whereas it tended to decrease in patients given calcium. The differences in the vertebral BMD changes between groups was highly significant (P less than 0.01). Radial BMD tended to increase in patients treated with AHBuBP, and to decrease in patients receiving calcium. The difference in the linear trends was statistically significant (P less than 0.05). The side effects of AHBuBP (a transient acute phase reaction in 3 out of 20 subjects) were slight and well-tolerated. A good effect of AHBuBP was observed also on back pain (P less than 0.05). We conclude that intermittent treatment with AHBuBP is capable of increasing spinal BMD and conserving radial BMD in postmenopausal osteoporosis and may represent a convenient therapeutic choice in this condition.