The effect of calcium supplementation on secondary hyperparathyroidism of old age depends on the intestinal absorption of the calcium preparation used. In order to test the bioavailability and clinical usefulness of a new calcium preparation, heated oyster shell-seaweed calcium (HOSS Ca), a randomized, prospective, double-blind comparison of HOSS Ca, calcium carbonate and placebo was carried out in 58 hospitalized women with a mean age of 82. Group A received 900 mg Ca/day Ca as HOSS Ca, group B 900 mg Ca/day as CaCO3 and group C placebo in addition to the basic hospital diet containing approximately 600 mg Ca/day. After 18 months, lumbar spine bone mineral density (BMD) measured by DPX was 106.1 +/- 3.5% (mean +/- SEM) of the pretrial basal value in group A, 99.8 +/- 3.8% in group B and 90.9 +/- 3.4% in group C (A bu not B significantly higher than C). Midradial BMD measured by DPX was 99.3 +/- 1.3% in A, 94.8 +/- 4.0% in B and 85.5 +/- 6.2% in C. The ratio of whole body calcium content between the 12th and 18th month was 96.5 +/- 1.7% in A, 90.1 +/- 2.7% in B and 89.8 +/- 1.3% in C (A but not B significantly smaller than C). Final/baseline ratio of urinary Ca/creatinine was 91.7 +/- 11.1 in A, 111.4 +/- 18.9 in B and 125.2 +/- 12.6 in C.(ABSTRACT TRUNCATED AT 250 WORDS)