Paraneoplastic limbic encephalitis is a rare cause of subacute dementia. A patient with limbic encephalitis and small-cell lung carcinoma is reported in whom the onset of subacute cognitive impairment was obscured by concurrent Alzheimer's disease. MRI revealed increased T2 signal in medial temporal lobes which corresponded to inflammatory pathology demonstrated at necropsy. High titres of antineuronal antibody (type II, anti-Hu) were present at death. Direct immunofluorescent staining of necropsy tissue revealed IgG bound to most remaining neurons in the temporal lobe. Antineuronal antibody screening and MRI are important in detecting limbic encephalitis.