The reduced capacity of older skin to synthesize vitamin D(3) under the influence of ultraviolet light makes older persons at risk of vitamin D deficiency. The risk could even be increased in institutionalized persons owing to their lower sunshine exposure. It has been reported that an inadequate vitamin D level is associated with secondary hyperparathyroidism, increased bone turnover, and bone loss, which increase fracture risk. The objective of this study was to assess the prevalence of inadequate serum vitamin D levels in institutionalized, postmenopausal, osteoporotic women. Assessment of 25-hydroxyvitamin D [25(OH)D] was performed in 445 institutionalized, osteoporotic women from nine countries (Australia, Belgium, France, Germany, Hungary, Italy, Poland, Spain and UK). For each institutionalized woman, three age-matched, noninstitutionalized, osteoporotic controls were also included. Four cutoffs of 25(OH)D inadequacy were fixed: less than 80, less than 75, less than 50 and less than 30 nmol/l. Mean age was 79.7 years (standard deviation [SD] = 5.8) for the institutionalized women and 79.5 years (SD = 5.5) for the noninstitutionalized women (p = 0.45). Significantly fewer institutionalized women received vitamin D supplements (13.2 vs 24.0%; p < 0.0001). In women without vitamin D supplements, the level of 25(OH)D was significantly lower in institutionalized women (56.9 [SD = 23.9] nmol/l) compared with noninstitutionalized women (63.2 [SD = 22.0] nmol/l; p < 0.0001). In institutionalized women (without vitamin D supplements), the prevalence of 25(OH)D inadequacy was 10.4, 41.2, 80.3 and 84.2% when considering cutoffs of 80, 75, 50 and 30 nmol/l, respectively. In the control group, the prevalence was 2.7, 22.9, 74.4 and 81.7%, respectively. The prevalence of vitamin D inadequacy was significantly higher in institutionalized women when considering the 75, 50 and 30 nmol/l cutoffs but not when considering the 80 nmol/l cutoff. This study highlights a high prevalence of vitamin D inadequacy in institutionalized, osteoporotic women. Compared with age-matched osteoporotic controls, the prevalence of severe vitamin D inadequacy was substantially more important in institutionalized women. We believe that a greater awareness of the importance of vitamin D inadequacy is needed in order to address this public health problem.