Objective: Our objective was to know the extent to which a fall in bone turnover markers is influenced by serum 25-hydroxyvitamin D (25OHD) levels in patients on alendronate (ALN) treatment. Design, Participants, and Setting: A total of 140 postmenopausal osteoporotic women were ran-domized to receive either ALN or ALN plus 25OHD 3 (ALNVitD) over a 3-month period. Serum 25OHD, PTH, C-terminal telopeptide of type I collagen (CTX), and amino-terminal propeptide of type I collagen (P1NP) were measured at baseline and at the end of the 3 months. Results: 25OHD rose four times above baseline levels in the ALNVitD group, whereas no changes were seen in the ALN group. Administering ALN resulted in a significant decline in both serum CTX (53 24%) and P1NP (46 19%). After ALNVitD, the fall in CTX amounted to 61 20% (P 0.06 compared with ALN) and P1NP to 50 23% (P 0.35). When patients were divided into those below and above 20 ng/ml of baseline serum 25OHD, in those below, CTX decreased by 48 26% in the ALN group and by 61 17% in the ALNVitD group (P 0.015). For P1NP, the corresponding figures were 43 20 and 50 23% (P 0.2). In patients above 20 ng/ml, no differences were seen regarding CTX (58 21% decrease in the ALN group and 60 23% in the ALNVitD group; P 0.7) or P1NP (49 18 and 50 20%; P 0.9).