Eighty-nine patients with two-level adjacent lumbar spinal stenosis were randomized to receive one drain tube (n=43) or two drain tubes (n=46) after BE-ULBD. The primary outcome was the cross-sectional area (CSA) of POSEH on MRI at 72 hours. Secondary outcomes included postoperative leg-pain VAS, total drainage volume, and length of stay. Outcomes were assessed up to 72 hours for MRI (and clinically thereafter for complications). Overall, the results showed no between-group differences in cranial CSA or caudal CSA, but total drain output was higher with two tubes, with no differences in 72-h VAS or hospital stay. Two cases of symptomatic POSEH occurred—one per group—managed successfully with percutaneous drainage. These findings suggest that adding a second drain increases output but does not meaningfully reduce POSEH formation following BE-ULBD.