Recognition of children at greatest risk for urolithiasis may allow early detection or prevention of stone formation. We report clinical data from 196 children aged 0.9-15.9 years in whom renal ultrasound examination revealed hyperechogenic spots in renal calyces less than 3 mm in diameter. We called this finding "calyceal microlithiasis" (CM). There was a history of urolithiasis in 70.4% of patients in at least one first- or second-degree relative. Presenting symptoms were recurrent abdominal pain, dysuria, and hematuria, occurring alone or in combination. Hematuria was the presenting symptom in 41% of patients and was the only urinary finding in more than one-third. Hypercalciuria was present in about one-third and hyperuricuria in one-fifth of the patients. Of 29 patients who were followed for at least 2 years, 9 developed calculi 4-7 mm in diameter. CM possibly represents the first step in calculus formation. The finding of CM might explain a number of symptoms and signs that are often mild and non-specific, thus reducing invasive diagnostic procedures.