Serum vitamin D metabolites and other parameters of mineral metabolism were measured in 12 patients with anorexia nervosa. Serum concentrations of calcium, phosphate, albumin, alkaline phosphatase, parathyroid hormone, calcitonin, osteocalcin, and 24-hours calcium excretion were normal. Serum 25-hydroxyvitamin D (25OHD) concentration was similar in patients and normal subjects, whereas 1,25-dihydroxyvitamin D (1,25(OH)2D) levels were significantly reduced in patients (62 +/- 17 vs 82 +/- 17 pmol/l); p less than 0.05). The concentration of vitamin D-binding protein (DBP) in patients was normal, but serum binding capacity (Nmax) was diminished in anorectic patients (2.05 +/- 0.50 vs 2.53 +/- 0.51 mumol/l; p less than 0.05). The diminished serum binding capacity, in spite of normal concentrations of albumin and DBP, reflects the presence of qualitative rather than quantitative defects in serum transport proteins. Since the reduction in 1,25(OH)2D and serum binding capacity was quantitatively similar, it is likely that free 1,25(OH)2D levels would be normal.