We report the case of an encephalitis patient with anti-N-methyl-D-aspartate receptor (NMDAR) antibody positivity. Anti-NMDAR encephalitis is a relatively rare autoimmune disease. In our patient, the diagnosis of anti-NMDAR encephalitis was established as late as several months after the first manifestations of the condition. Further development demonstrated that the patient probably suffered from a paraneoplastic form of the disease, although the presence of an underlying tumour was not detected by the available imaging methods at the time of diagnosis. The case is a rarity since the disease usually affects females, and only 5% of adult male patients have a paraneoplastic aetiology.