In patients with mental disorders, psychic symptoms as well as effects of antipsychotic drugs may mask the symptoms and signs of physical disorders which they have and impede the early diagnosis. As for acute appendicitis, not a few patients with mental disorders visit a surgeon, when the symptoms of the disease are greatly modified due to delayed diagnoses. Clinical and pathological findings were studied in eighty patients with mental disorders who underwent operations for acute appendicitis in Tokyo Metropolitan Matsuzawa Hospital (the mental group). Ninety-five patients without mental disorders undergone operations for acute appendicitis in Yuai Hospital were served as control (the control group). Although right-lower abdominal pain was the most predominant symptom in both groups, patients of the mental group showed more variegated symptoms than the control. The ailing period between the onset of appendicitis and operation was longer in the former group than in the latter. General anesthesia was used for the operation more frequently in the mental group. The median or paramedian incision was selected for laparotomy more often in this group. The incidences of the postoperative complications were also higher in the mental group probably due to the delayed diagnosis. A careful and constant observation would be necessary for the early detection of the physical disorders in patients with mental disorders who can not tell others their abnormalities.