The ultrahistopathology of six patients with psoriatic oral mucosal membrane lesions is described. Four of the patients had tongue lesions (geographic or fissured tongue) and two had lesions of the buccal mucosa. The same features were found in both types of lesions. The most prominent feature was the large number of neutrophils. Neutrophil pustules caused flattening of the keratinocytes, resulting in a sponge-like network of epithelial cells. Other features were the dilated fenestrated capillaries surrounded by neutrophils and oedema of the upper lamina propria, basal lamina gaps, basal spongiosis and keratinocyte activation. Psoriasis lesions of the oral mucosa belong to the group of diseases characterized by epithelial non-bacterial pustules.