The loss of benefit from intrathecal baclofen, with increased spasticity and a discrepancy between the residual and the calculated volume content (underinfusion), made us suspect dysfunction of the intrathecal baclofen infusion in a paraplegic patient. Although all possible usual checking methods were used, no failure in the drug administration device (DAD) could be found. Despite some benefit from increasing the daily dose of baclofen, it has not been possible to control the spastic symptoms. Surgery was therefore decided upon, and a small leak at the end of the catheter tip was discovered. Surgeons and physicians should be aware that checking methods of DAD cannot exclude failure of the system. Their errors and limitations are discussed.