Hydatid cysts can be found in almost every structure in human body. Thus they can mimic numerous pathologies. Serological analyses are the best assistants in the diagnosis of hydatid cysts. But as serological investigations have limited sensitivity and specificity, they might misdirect our diagnosis. Lungs are the most common involved organ in this parasitic infection. But the brain, muscles, kidneys, bones, heart, pancreas, and rarely the retroperitoneum have also been reported to be involved. An unusual case of a sero-negative hydatid cyst located in the retroperitoneum presenting with lower urinary tract symptoms is reported in this paper.