OBJECTIVE Intraluminal meconium calculi are a rare cause of neonatal abdominal calcifications in patients with anorectal malformations. To investigate their pathogenesis, we performed infrared spectroscopic analysis of meconium-calcified lesions. METHODS Meconium calculi were collected from the colostomy in a newborn patient with imperforate anus and rectourethral fistula. The potassium bromide method was employed to obtain the infrared absorption spectrum of the meconium calculi. RESULTS The wavelength pattern of the meconium calculi exhibited 4 specific peaks at 1570, 1390, 1105, and 1005 cm(-1) between 22% and 45% transmittance values. The unique absorption spectrum exclusively indicated ammonium hydrogen urate (C(5)N(5)O(3)H(7)), having the combined constituents of ammonium and uric acid. CONCLUSIONS These results suggest that the intraluminal meconium calculi were originally derived from meconium and fetal urine. The stasis of meconium passage and fetal urine mixing through the rectourethral fistula in a low-pH condition was deduced to be the main cause of this rare stone formation.