OBJECTIVE and BACKGROUND Old people in residential care are at the highest risk of any group for hip fracture. This may relate to their high prevalence of hyperparathyroidism. There are few data, however, on relationships with serum parathyroid hormone (PTH) in these individuals. This study therefore examined complex associations with serum PTH in nursing home and hostel residents.DESIGN Cross-sectional analysis.PATIENTS One hundred and forty-three nursing home and hostel residents of median age 84 years.MEASUREMENTS Serum PTH, 25-hydroxyvitamin D (25OHD), 1,25-dihydroxyvitamin D (1,25-(OH)(2)D), plasma creatinine, phosphate, calcium, albumin, Bsm-1 vitamin D receptor genotype, age, weight and use of frusemide or thiazide.RESULTS The statistical models determined accounted for half the interindividual variation in serum PTH. Heavier weight was associated with both the prevalence of secondary hyperparathyroidism and the serum concentration of PTH. Novel interactions with serum PTH were identified between: weight and 25OHD; 25OHD and phosphate; and phosphate and thiazide diuretic use. Plasma phosphate was associated with PTH independently of calcium and 1,25-(OH)(2)D. There was no independent association between PTH and nuclear vitamin D receptor genotype.CONCLUSIONS Heavier weight is associated with both the prevalence and severity of secondary hyperparathyroidism and consistent with animal models of secondary hyperparathyroidism, phos-phate may relate to serum PTH independently of 1,25-(OH)(2)D or calcium.