Background: Besides inducing behavioral unresponsiveness, a key goal of anesthesia is to prevent the experience of surgery (connected consciousness) by inducing either unconsciousness or disconnection of consciousness from the environment. In pursuing this key goal, avoiding the risk of postoperative cognitive dysfunction is also necessary. In this case report we relate electroencephalography waveform characteristics during intraoperative bispectral index monitoring to long-term neurocognitive decline. Case presentation: We describe the case of two 68-year-old homozygous twins who underwent the same general anaesthesia regimen, but had different results on neurocognitive testing 12 months postoperatively. Neurocognitive tests (Montreal Cognitive Assessment, Trail Making Test A and B, Digit Span Test, and Frontal Assessment Battery) were performed in three different times: the day before surgery, the third day postoperatively, and 12 months after surgery for follow-up. During anaesthesia, connected consciousness and anaesthesia depth were monitored using the isolated forearm technique and bispectral index monitoring. The mean value of bispectral index was lower in one twin (S1) than in the other (S2). Moreover, the same twin who presented lower values of bispectral index had two episodes of burst suppression, whereas the other had none. Regarding the neurocognitive evaluations, both twins did not show significative differences between the tests performed before and after surgery, but an important difference was founded at the 12-month follow-up, when only one twin (S1) showed distinctly worse test scores than previous, denoting a significant cognitive decline. Conclusions: This case-report suggests that bispectral index electroencephalography waveform characteristics during intraoperative bispectral index monitoring may be useful in detecting brain’s susceptibility to general anaesthesia and in predicting long-term neurocognitive decline.