We examined the impact of daily supplementation on vitamin D deficiency, function, and falls in female long-term care residents. Initial vitamin D deficiency was associated with greater functional decline and increased fall risk despite guideline-recommended supplementation, highlighting the importance of preventing vitamin D deficiency in frail elderly. Institute of Medicine (IOM) guidelines recommend 800 IU vitamin D daily for older adults and maintaining serum 25-hydroxyvitamin D [25(OH) D] above 20 ng/ml for optimal skeletal health. The adequacy of IOM guidelines for sustaining function and reducing falls in frail elderly is unknown. Female long-term care residents aged a 65 enrolled in an osteoporosis clinical trial were included in this analysis (n = 137). Participants were classified based on baseline 25(OH) D levels as deficient (30 ng/ml, n = 71). Deficient women were provided initial vitamin D repletion (50,000 IU D-3 weekly for 8 weeks). All were supplemented with 800 IU vitamin D-3 daily for 24 months. Annual functional assessments included Activities of Daily Living (ADLs), Instrumental ADL (IADL), physical performance test (PPT), gait speed, cognition (SPMSQ), and mental health (PHQ-9). We used linear mixed models for analysis of functional measures and logistic regression for falls. Daily supplementation maintained 25(OH) D levels above 20 ng/ml in 95% of participants. All groups demonstrated functional decline. Women initially deficient had a greater decline in physical function at 12 (IADL -2.0 +/- 0.4, PPT -3.1 +/- 0.7, both p < 0.01) and 24 months (IADL -2.5 +/- 0.6, ADL -2.5 +/- 0.6, both p < 0.01), a larger increase in cognitive deficits at 12 months (1.7 +/- 0.4: p = 0.01) and more fallers (88.5%, p = 0.04) compared to those sufficient at baseline, despite supplementation to sufficient levels. IOM guidelines may not be adequate for frail elderly. Further study of optimal 25(OH) D levels for maintaining function and preventing falls is needed.