We investigated the effects of dietary oligofructose and Ca on bone structure in ovariectomized rats, using microradiography and histomorphometry. Ninety-six animals were allocated to seven experimental groups: G1, sham-operated; G2-G7, ovariectomized. Semi-purified diets containing 5 g Ca/kg (recommended content) without oligofructose (G1, G2) or with 25, 50 or 100 g oligofructose/kg (G3, G4, G5) or 10 g Ca/kg (high content) without oligofructose (G6) or with 50 g oligofructose/kg (G7) were fed for 16 weeks. At the recommended level of Ca, high oligofructose (G5) increased femur mineral levels in ovariectomized rats, while medium oligofructose did so at high Ca. Increasing Ca in the absence of oligofructose did not increase femur mineral content. Trabecular bone area (%) analysed in the tibia was 10·3 (SEM 1·2) (G1), 7·7 (SEM 0·6) (G2), 9·3 (SEM 0·7) (G3), 9·4 (SEM 1·0) (G4), 9·5 (SEM 0·7) (G5), 10·2 (SEM 0·8) (G6), and 12·6 (SEM 0·8) (G7). At the recommended level of Ca, 25 g oligofructose/kg prevented loss of trabecular area due to increased trabecular thickness, while 50 or 100 g oligofructose/kg increased trabecular perimeter. At high Ca, oligofructose prevented loss of bone area due to increased trabecular number but similar thickness (G7 v. G6). When Ca was raised in the presence of oligofructose (G7), trabecular area and cortical thickness were highest, while loss of trabecular connectivity was lowest of all groups. At the same time, lumbar vertebra Ca was higher; 44·0 (SEM 0·8) (G7) compared with 41·6 (SEM 0·8) (G2), 41·4 (SEM 0·7) (G4), and 40·5 (SEM 1·0) mg (G6). We conclude that ovariectomy-induced loss of bone structure in the tibia was prevented but with different trabecular architecture, depending on whether dietary Ca was increased, oligofructose was incorporated, or both. Oligofructose was most effective when dietary Ca was high.Oligofructose: Calcium and phosphorus balance: Osteoporosis: Trabecular structure:Bone qualityCertain groups of carbohydrates are not digested by gastrointestinal enzymes but are fermented by the microbial flora in the large intestine (Roberfroid et al. 1998). Non-digestible oligosaccharides (NDO), including inulin, oligofructose and fructo-oligosaccharides, and galacto-oligosaccharides, as well as lactulose, maltitol and resistant starch, were studied for their gastrointestinal effects. Several effects of NDO on the colonic milieu, composition of the microflora, production of microbial metabolites (Gibson et al. 1995) and mineral metabolism have been described (for reviews, see ScholzAhrens et al. 2001ScholzAhrens et al. , 2002. A potential for benefits to the host's health was postulated. In this regard, consensus was achieved that there is promising evidence for positive effects on mineral metabolism and thus a potential for prevention of osteoporosis (van Loo et al. 1999). Most of the scientific evidence for the effects of NDO is based on the results of experiments with rats, where these carbohydrates increase the availability of Ca (Delzenne et al.