A 64-year-old woman was referred to our hospital for management of an ovarian tumor. Abdominal computed tomography and magnetic resonance imaging revealed a dermatoid cyst of the ovary and a bladder tumor. Transurethral resection of the bladder tumor was performed. Histopathological examination of the tumor revealed non-Hodgkin's lymphoma of the mucosa-associated lymphoid tissue MALT type. The patient received radiotherapy for the bladder and had a complete response. Nineteen months later, gastrointestinal endosopy revealed the presence of a mass lesion in the stomach. Histopathological examination of biopsy specimens from this tumor indicated the same tumor as that in the bladder as they showed identical IgH gene rearrangement. Because of the detection of evidence of Helicobacter pylori (HP) infection in the gastric mucosal biopsy specimens, the patient was administered HP eradication therapy, but, the tumor persisted. After radiotherapy, the stomach tumor disappeared. Since then she remains without evidence of local recurrence or relapse.