A case is presented of 41 year old patient with a very rare form of clinical manifestation of adenocarcinoma with a large mucinous ascites with abdominal distention and a massive left hemithoracic content with same characteristics. Two years prior the patient suffered from acute abdomen and underwent apendectomy with slow wound healing and a stomach edema 6 months later with a finding of smaller changes on the left side of the thorax. The patient was hospitalized when he already had an advanced stage of the disease. The diagnosis of adenocarcinoma was reached with percutaneous thoracic biopsy and finding of mucinous malignant cells. Gelatinoid ascites was confirmed on CT an echography with the same finding in the thorax. Systemic chemotherapy was initiated without major effect followed by intrapleural and intraabdominal instillation of hyaluronidase in combination with cytostatics. The course of the disease and treatment effects were followed echographically and on CT. According to available literature this is the first case report of a patient with mucinous adenocarcinoma and a presentation of pseudomyxoma of the thoracic and abdominal cavities.