The study reported in this paper was aimed to develop an accurate, specific, and precise high-performance thin-layer chromatography (HPTLC) method for the determination of oleandrin in Nerium oleander (Linn.) extracts as well as biological samples of an intoxication case for the first time. Also, extraction, isolation, and purification methods of oleandrin from the sticky juice of Nerium plant leaves were presented. The retention factor (R-F value) of oleandrin was found to be 0.24 +/- 0.01. The linearity range was between 2 and 75 ng spot(-1) (r = 0.9993), the limit of detection was 2.2 ng spot(-1), the limit of quantification was 6.7 ng spot(-1), and the recovery in urine and serum was found 89% and 83%, respectively. Isolated compounds from both plant material and biological samples were also applied to mass spectrometry (MS). The reported HPTLC method was sensitive, rapid, and suitable for the analysis of oleandrin and was applied to a non-fatal case of oleander poisoning. Oleandrin was detected in urine and serum samples of the intoxicated patient at concentrations of 3.2 and 8.4 ng mL(-1), respectively. Peak purity of the isolated compound was checked by electrospray ionization-mass spectrometry (EI-MS) and introduced to HPTLC compound library. To the best of our knowledge, this study is the first published HPTLC method and also presents the evaluation of both clinical and toxicological aspect of an oleander posioning case. It can be successfully applied for the determination of oleandrin in clinical and forensic researches as well as in phytochemical investigations.