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Background Intake of melamine can cause renal and ureteral lithiasis in infants and children. The present study aimed to understand the value of ultrasonography in the diagnosis of renal and ureteral lithiasis in infants and young children caused by melamine, and the characteristics of ultrasonograms of melamine-associated calculi.Methods Ultrasonographic examination on the urinary system was performed for 28 332 children who ingested milk powder that was possibly tainted with melamine; 395 of the children were diagnosed by ultrasonography as having urinary calculus, and 231 cases had lump-like calculi and 164 cases had sand gravel-like calculi. The features of the calculi, the sites of obstruction and the status of hydronephrosis and hydroureterosis were analyzed. Ultrasonographic reexamination was performed for 116 patients and the findings were compared with those of the first ultrasonography, and the short-term expulsion of the calculi was evaluated.Results Most of the 395 patients with urinary lithiasis, except for those who developed renal failure, had no symptoms. The whole profile of the calculi could be seen in most of the cases, because the echogenicity of the calculi with no sharp or absent acoustical shadowing, was weaker as compared with those from calcium-containing calculi. Comet tail sign could be seen behind the echogenicity of single gravel calculus. The rate of diagnostic consistency of ureteral lithiasis between the ultrasound and the results of clinical observation (including stones expelled spontaneously or after cystoscopic intervention) in 51 cases for 76 ureters was 100%. Percutaneous renal biopsy was performed for one case, and histopathological examination showed flocculent, fine strip-like, ellipse and circular deeply stained sand gravel-like material in the renal tubules, and the circular calculi were found to be attached to the walls of the tubules. Chemical analysis of the calculi expelled from 12 cases showed that the main contents of the calculi were uric acid and melamine. Short-term ultrasound reexamination in 116 patients showed that gravel-like calculi disappeared in 80.4% of the cases; in 26 non-hospitalized cases who had lump-like calculi without hydronephrosis or hydroureterosis, none of the lump-like calculi were expelled.Conclusions The ultrasonograpic findings of urinary calculi in children caused by ingestion of melamine-tainted milk powder have a certain features as compared to the calculi containing calcium. Careful ultrasound examination can avoid missed diagnoses of ureteral calculi. Most of the gravel-like calculi can be expelled within a short period of term, while lump-like calculi can hardly be expelled. Ultrasonography is an accurate and reliable method of diagnosis of urinary calculus caused by melamine intake in children and it can be used as diagnostic method of choice although abdominal X-ray plain film can also visualize some of larger calculi.