Idiopathic infantile hypercalcemia (IIH) is a rare cause of hypercalcemia in the 1st year of life and was initially considered part of a spectrum encompassing vitamin D intoxication, Williams syndrome, and idiopathic hypercalcemia. Identification of the gene for Williams syndrome now allows a clear separation of IIH from Williams syndrome. The inheritance and pathogenesis of IIH remains largely unknown, with only sporadic cases reported to date. This report describes a family with two siblings with IIH. The pedigree is consistent with autosomal recessive inheritance, but more complex inheritance is suggested by the occurrence of hypercalciuria in a number of family members. Although one affected patient demonstrated elevated 1,25-dihydroxyvitamin D(3) levels, no conclusions regarding the pathogenesis of this condition could be drawn.