Idiopathic hypercalciuria (IHC) has been reported mainly in children with hematuria in the 1980s and early 1990s, when renal sonography was just becoming routine. The presence of microcalculi, i.e., of hyperechogenic spots < 3 mm in diameter in renal calyces, was not taken into account in those studies. We attempted to outline clinical presentation and natural course of IHC not only in children with hematuria, but also in those with dysuria and/or recurrent abdominal/flank pain and a family history of nephrolithiasis, taking into account the finding of microcalculi. We analyzed retrospectively the data at diagnosis from 74 consecutive children aged 2.4-18 years (mean 8.6) with IHC (calciuria 4.1-15.1 mg kg-1 24 h-1, mean 6.1) and the outcome of 30 of them who were followed > or = 1 years (mean 3.2) with no specific therapy. At diagnosis, 38 patients (51%) had no hematuria, 42 (57%) had microcalculi and four (5%) had calculi. Of the patients with normal urinalysis, 71% had microcalculi or stones. The subjects with microcalculi and those with stones were significantly older than those without microcalculi and stones (P = 0.004 and 0.007). A normal urinalysis at our evaluation and a history of abdominal/flank pain were significantly more frequent in patients with microcalculi than in those without (P = 0.02 and 0.0001, respectively). During the follow-up, four of 30 patients formed stones 1-3 years after first diagnosis of IHC. More than half of children with IHC have microcalculi. The risk of formation of microcalculi or stones increases with age. The lack of hematuria does not exclude the presence of microcalculi or calculi. Hypercalciuria has to be suspected in children with dysuria and/or recurrent abdominal flank pain and a family history of nephrolithiasis, even when they have no hematuria.