The aim of this placebo-controlled study was to determine whether once-weekly clodronate could prevent osteoporosis in patients with arthritis at the start of corticosteroid therapy. One hundred sixty-three patients, 18 to 90 years of age, with rheumatoid or psoriatic arthritis, were randomly assigned to receive either clodronate (100 mg im/week) plus calcium and vitamin D (1000 mg and 800 UI, respectively) or calcium and vitamin D alone. Patients had started therapy with prednisone or its equivalent within the previous 100 days and had bone mineral density <2.5 SD below mean young normal values at the lumbar spine or femoral neck. The primary outcome was the difference between the two treatment groups at months 12, 24, 36, and 48 in the mean percentage change from baseline in the bone mineral density of the lumbar spine, femur (neck and total), and total body. Secondary measurements included changes in the stiffness index evaluated by ultrasound measurements and the rate of new vertebral fractures. The bone density and stiffness did not change significantly in the clodronate plus calcium and vitamin D group, whereas it declined significantly in the calcium plus vitamin D group. The difference between treatment groups at 48 months in the mean change from baseline was 8.78 +/- 1.4% for the lumbar spine (P < 0.01), 7.31 +/- 1.12% for the femoral neck (P < 0.01), 7.92 +/- 1.93% for the trochanter (P < 0.01), 8.39 +/- 1.80% for total femur (P < 0.01), 6.94 +/- 1.09% for total body (P < 0.01), and 9.38 +/- 2.21% for stiffness of os calcis (P < 0.01). Depending on the skeletal regions evaluated, 85 to 98% of patients treated with clodronate had a densitometric change lower than the lowest significant densitometric difference. One hundred percent of patients treated with calcium plus vitamin D had a densitometric decrease greater than the lowest significant difference. The relative risk of vertebral fractures and multiple vertebral fractures in the clodronate group compared to the calcium plus vitamin D group was 0.63 (0.35-0.98, 95% CI) and 0.25 (0.15-0.91, 95% CI), respectively. We concluded that pulsatory administration of im clodronate once weekly is a safe therapy for preventing corticosteroid induced osteoporosis in patients with arthritis.