The authors evaluated the size of the disc herniation with magnetic resonance imaging (MRI) before and after surgery in patients undergoing automated percutaneous lumbar discectomy (APLD) and compared the MRI findings with the early clinical outcome. This study includes 20 consecutive patients with a contained lumbar disc herniation. Sequential MRI were performed immediately before and after surgery, on the day of surgery, and 6 weeks after surgery. The development of pain, nerve root tension sign (SLR), and neurological findings were analyzed, as was the need for subsequent open surgery. There was no significant difference in the maximum protrusion of the disc herniation between the three measurements. The sciatic pain improved significantly on the first day after surgery but not at 1 week or 6 weeks after surgery. The SLR was reduced significantly after surgery and at 1 and 6 weeks after surgery. There was no correlation between the MRI findings and the early clinical outcome. Seven patients needed subsequent open surgery. The findings indicate that the effect of APLD is not mediated by reducing the size of the disc herniation. In this small series of patients, APLD was ineffective in the treatment of a contained lumbar disc herniation. There was no correlation between the MRI findings and the early clinical outcome.