Muscle weakness is an important complication of advanced pulmonary disease and it is associated with reduced functional activity and lower survival. Vitamin D may be involved in muscle function. The aim of this study was to investigate determinants of calcidiol (25-hydroxycholecalciferol, the major circulating indicator of vitamin D) status and associations between vitamin D metabolites and muscle function in relation to nutritional depletion. Fifty-two percent of the underweight patients (n = 42) and 55% of the normal-weight ones (n = 29) had vitamin D deficiency (< 37.5 nmol/L). The resulting models of linear regression showed that, for the calcidiol model, 24.7% of the variation for calcidiol was explained by fat mass index, vitamin D intake, and FEV(1)/FVC. The results further suggested that vitamin D intake was a stronger predictor of calcidiol status in the underweight patients than in the normal-weight ones. In the resulting models for 6-minute walking distance, calcidiol was a significant predictor, which tended to be more marked in the underweight patients than in the normal-weight ones. Low serum calcidiol concentration was associated with fat mass, lung obstruction, and low intake of vitamin D, especially in the underweight patients, and calcidiol was a predictor of walking distance.