The indication of bisphosphonates in hypercalcemia is fully accepted, the long term therapy with bisphosphonates is still controversial. The aim of our study was to evaluate the influence of clodronate on the bone density of myeloma patients. Twenty patients were included in the study. Clodronate is administered in the total dose of 3,000 mg, which is delivered in 4-6 hour infusions, 600 mg/day, once in tree 3 months. The effect of clodronate on bone density is evaluated by CT-densitometry in a period of 6 months. At the beginning of May 1994, 15 patients had completed at least two estimations of bone density. The amount of hydroxyapatite in these six months increased in 9 patients, in one of them there was no change and in 4 of them decreasing bone density was detected. The mean bone density before the administration of clodronate was -2.6 SD (standard deviation of European standard of bone density for age and sex). After 6 months of therapy the bone density increased to -2.3 SD. The mean amount of hydroxyapatite in spongiosa was raised from the mean value 32.71 mg/ml before clodronate administration to 38.91 mg/ml after the 6 month treatment period. The mean increase in calciumhydroxyapatite in trabecular bone mass was 6.2 mg. Clodronate contributed to the amelioration of bone pain in the majority of patients, but this effect is difficult to evaluate because of other treatment modalities administered concomitantly. The tolerance of clodronate was good. No impairments of renal function or other adverse effects were observed. Only in 2 patients the decrease in calcium concentration caused slight tetania. Therefore close monitoring of the calcium level is recommended and in the case of its decrease below the physiological level peroral substitution of calcium was started.