Deficiency of vitamin B6 in rats may result in defective bone formation, possibly due to decreased activity of the enzyme ornithine decarboxylase which requires pyridoxal-5'-phosphate (PLP) as a co-factor and is responsible for production of intracellular putrescine, a metabolic regulator. We studied 3 groups of patients (62 fit ambulant out-patients, 21 elective arthroplasty patients, and 20 hip fracture patients) and assayed their PLP status by high performance liquid chromatography. The reference range derived from the out-patients was 13-106 nmol/L. 3 of the arthroplasty group and 10 of the fracture group had serum PLP concentrations less than 13 nmol/L (P < 0.01). We conclude that PLP may be an etiologic factor in hip fracture by virtue of its role in the activity of a key regulatory protein.