The urachus is an embryonic structure that persists after birth in some individuals and can cause various problems. We report a case of an inflamed urachal cyst filled with a thick yellow fluid and several calculi in a woman with a 1‐month history of dysuria. Physical examination revealed a fist‐sized tumor located infraumbically in the midline. The patient's erythrocyte sedimentation rate was elevated; the results of all other routine laboratory studies were normal. Sonography showed a regularly shaped, ovoid, hypoechoic cystic area in the abdominal wall measuring 8 × 4 × 3 cm and containing several hyperechoic masses associated with acoustic shadowing. The wall of the cyst was inhomogeneous, and a thin hypoechoic linear tract linked the superior aspect of the mass to the umbilicus. The results of excretory urography, voiding cystography, and cystoscopy excluded an abnormality of the urinary system. A urachal cyst was diagnosed, and the mass was surgically removed. The surgical specimen was sent for histopathologic analysis, which confirmed the diagnosis. © 2002 Wiley Periodicals, Inc. J Clin Ultrasound 30:253–255, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jcu.10056