Twenty patients with mild Paget's disease of bone were given either 20 (10 patients) or 40 mg alendronate daily for 6 months. The 20-mg dose was well tolerated, but in 3 patients on 40 mg/d alendronate, the treatment was withdrawn after 3-5 months because of gastric and oesophageal disturbances. Urinary hydroxyproline excretion fell within the first month to 77 +/- 5% (SD) and to 47 +/- 5% of pretreatment values in the 20- and 40-mg dosing group, respectively (p < 0.001 between group comparison). The serum alkaline phosphatase fell more slowly with the maximum suppression of disease activity reached at 4 months, when it attained a plateau in both groups of patients. However, the decrease in serum alkaline phosphatase was significantly more pronounced in the patients treated with 40 mg/d tablets (50 +/- 10% of pretreatment values) than in those given 20 mg alendronate per day (76 +/- 9% of initial value), in none of whom a disease remission was observed. It appears, therefore, that while 20 mg/d oral doses of alendronate are insufficient, 40 mg/d are associated with a high incidence of side effects. Furthermore, the suppression of disease activity depends on the dose of bisphosphonate given daily or over a short period of time and lower doses cannot be compensated by a longer duration of the treatment course.