Paget's disease of bone (PDB) is a chronic disorder characterized by localized bone regions with excessive bone turnover. Although oral risedronate (17.5 mg daily for 8 weeks) was recently approved in Japan, its efficacy is not well understood. We retrospectively examined the efficacy of oral risedronate in PDB patients in a clinical setting. Eleven patients whose serum alkaline phosphatase (ALP) level exceeded the upper limit of the normal range were treated. Patients whose ALP levels normalized and remained so for 12 months after therapy initiation were defined as responders. Treatment was repeated if bone pain recurred or if serum ALP levels increased at least 25% above the nadir. Six patients (55%) were responsive to the therapy. A higher prevalence of skull lesions, higher serum calcium levels at treatment initiation and antecedent treatments of bisphosphonates were predictors of resistance against the therapy. Fresh frozen serum samples obtained from some treatment sessions were evaluated for metabolic bone markers such as bone-specific ALP (BAP), type I procollagen N-terminal pro-peptide (PINP), N-treminal crosslinking telopeptide of type I collagen and C-treminal crosslinking telopeptide of type I collagen (CTX). A significant reduction of P1NP preceded that of serum ALP levels in the responders, which was followed by a similar occurrence for BAP and osteocalcin (BGP) levels. A temporary decrease in CTX levels was noted. No significant changes in markers (including ALP level) were observed in non-responder and repeat-treatment groups. P1NP levels may be more useful than ALP levels in assessing treatment efficacy. Repeat treatment effectiveness for the repeat-treatment group was limited.