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The intestinal absorption of calcium is one of the most important processes regulating calcium metabolism. Its measurement requires the estimation of dietary and faecal calcium over a period of one week, but this procedure is frequently liable to error. The availability of two isotopic procedures contributed to the successful pursuit of measuring intestinal absorption of calcium. In the first procedure the indirect assessment of calcium absorption is based on the kinetic study of plasma radioactivity curve after an intravenous dose of radiocaldium at the same time with a calcium balance study over a period of 6 days, In the second isotopic procedure the calcium absorption is directly calculated from the plasma radioactivity and residual faecal activity following an oral dose of radioactive calcium. We have investigated by the two methods 25 patients (one normal subject and 24 affected by a variety of clinical disorders of calcium metabolism). The excellent agreement, statistically significant, between the data obtained by the two methods suggests that both procedures are adequate to assess intestinal absorption of calcium in man.