Recent dietary life involves frequent opportunities for the ingestion of purified, processed food products and preserved foods, and it has been pointed out that the current dietary mineral intake strongly tends toward nutritional imbalance. The Ryukyu Islands yield coral which contains calcium and magnesium in a content ratio of about 2 to 1, with their approximate contents of 20 and 10%, respectively. In this report, the calcium absorption from the ingestion of crackers into which the coral powder was incorporated (coral-added crackers) and that from ingestion of calcium carbonate-added crackers was comparatively assessed. Twelve healthy adult volunteers (6 men and 6 women) ingested coral-added crackers (calcium content: 525 mg) and calcium carbonate-added crackers (ditto) once each alternately on a cross-over design with a wash-out period of 3 d between the regimens. The study also included controls receiving neither cracker. The degree of intestinal absorption of calcium from coral-added crackers and that from calcium carbonate-added crackers was evaluated in terms of increment in urinary calcium excretion per dL of glomerular filtrate (GF) (difference between coral calcium and calcium carbonate) and increase in urinary calcium excretion per milligram creatinine (difference from control value). The increment in urinary calcium excretion per dL of GF during the latter half of the observation period after the ingestion of coral-added crackers was significantly greater than that during the latter half of the observation period after ingestion of calcium carbonate-added crackers (p = 0.039, paired t-test). A significant difference (from control value) in the increase of urinary calcium excretion per milligram creatinine was also observed (p = 0.0008). The present data, though from a relatively few study subjects, suggest that the calcium of coral origin is better absorbed from the intestine than calcium of calcium carbonate origin on the average.