The efficacy of a monofluorophosphate-calcium combination (MFP-Ca) in increasing lumbar bone mineral density (BMD) was assessed in a prospective double-masked study. Patients (n = 35), who had been treated for 1 year or more with prednisone-equivalent doses > or = 7 mg/day for asthma or other respiratory diseases, were randomly assigned to receive twice a day, for 2 years, either one MFP-Ca tablet [100 mg sodium monofluorophosphate (13.2 mg F-) + 500.5 mg Ca2+] or one Ca tablet (500.5 mg Ca2+). BMD was measured from L2 to L4 using a dual photon absorptiometer. The eligible patients (7 premenopausal women, 21 men), who had no previous vertebral fractures and were aged 46.5 (21-65) years, had received 18 (7.5-60) mg prednisone-equivalent/day and had a mean lumbar BMD of 0.917 +/- 0.141 g/cm2 at baseline (MO); in these 28 patients, the mean increase in lumbar BMD at final assessment was significantly greater in the MFP-Ca group (p = 0.05; Mann-Whitney). There was also a significant difference after 2 years between the two groups (p = 0.05, ANOVA) in favour of MFP-Ca, with an increase in lumbar BMD of 11% (MFP-Ca) compared with 1% (Ca); thus, with MFP-Ca, lumbar BMD increased by an average of approximately 5.5%/year. There was no statistically significant difference between the two groups in doses of corticosteroids used during the 2 study years, rate of vertebral fractures, or frequency of side-effects (which were all minor). No bone fissure was observed. Thus, the daily dose of 200 mg monofluorophosphate (26.4 mg F-) combined with 1 g Ca2+ in patients with long-term corticosteroid-treated respiratory diseases appears to be a safe and efficient way of increasing lumbar BMD, suggesting that its use should be further studied in corticosteroid-induced osteoporosis.