Concentrations of both serum calcium, adjusted for albumin, and serum magnesium of siblings from 23 families were found to cluster around different concentrations within the normal reference interval. Variation between families accounted for 37% of the total variation in calcium and 28% of that in magnesium. A disturbance of serum calcium or magnesium homeostasis in an individual resulting in an altered serum concentration that remains within the reference interval might be recognized by examining values in siblings.
