We analysed some genetic and environmental factors influencing renal (RH) and absorptive hypercalciuria (AH) the main subtypes of idiopathic hypercalciuria (IH). Their distinction is essential in the prevention and treatment of urolithiasis. Twenty-one children admitted for renal stones had IH (8 RH and 13 AH). Their families were investigated with the participation of a total of 68 family members including 62 first-degree relatives. They were all normocalcaemic. Their urinary calcium excretion was measured on at least two occasions, and they underwent a calcium restriction--calcium loading test. Each subject was interviewed with reference to medical history, drug administration, social conditions, and nutritional habits. Pedigree analysis suggested a higher rate of familiarity and autosomal dominant inheritance of RH whereas no such pattern was found among the relatives of patients with AH. The interviews disclosed a distinct preference for calcium-rich foods in subjects with AH, while RH and normocalciuric individuals preferred a low-calcium diet. We conclude that the subtypes of IH are genetically different. RH is most probably inherited as an autosomal dominant trait (or has at least an autosomal dominant form), whereas AH is more likely to be associated with nutritional factors.