Background: Implantable intrathecal drug delivery systems (IDDS) are basic tool enabling chronic intrathecal pharmacotherapy. Lack of expected clinical results of IDDS therapy necessitates search for the cause with the help of diagnostic imaging methods among other things. Beside radiological techniques, it is also possible to visually assess IDDS systems by nuclear medicine methods. In this study we assess utility of radioisotopic methods in differential diagnosis of failure of therapy with IDDS systems. Material/Methods: Scintigraphic studies were performed in selected patients with neurological diseases associated with spasticity, who had IDDS system implanted and were unable to maintain satisfying clinical effect of inrathecally infused baclofen. After emptying the IDDS system of the drug, radiotracer (99mTc-DTPA) solution was injected into the pump reservoir. Subsequently, a series of scintigraphic images was registered, demonstrating passage and distribution of the infused radiotracer. Results: In all investigated cases, scintigraphic study resulted in acquiring relevant additional diagnostic information. Normal or disrupted distribution of radiotracer in spinal canal allowed for a diagnosis drug resistance or demonstrated presence of arachnoid adhesions respectively. Early appearance of radiotracer in blood was considered a proof of leak. Our examinations had decisive influence on further patient treatment, allowing for diagnosis of drug resistance in one patient or complication related to IDDS system in three other cases including breakage of a catheter, pump malfunction and arachnoid adhesions. Conclusions: Scintigraphic methods carry significant amount of information facilitating final diagnosis of the cause of IDDS therapy failure. They should become an important element complementing the diagnostic strategy in patients with suspected failure of intrathecal drug administration systems. Interpretation of radioisotopic studies, since they are purely functional, must be performed in strict relation to clinical data and radiological examinations as they carry indispensable, basic information regarding morphology.