Mucinous colorectal cancer with signet ring cell aspects is a rare form of adenocarcinoma representing about 2-5% of large bowel neoplasms. The tumor frequently presents with late-stage or peritoneal spreading. It can often affect young patients and diffusely infiltrate through all layers of the intestinal wall. Local recurrence and distant metastases are common in spite of surgical operation and adjuvant treatment, conditioning a poor prognosis. At the present, early diagnosis and complete resection are the most important approaches to improve the outcome. In our report we describe a case of a 41-yr-old patient with very aggressive untreated metastatic colorectal signet ring cell carcinoma. The fulminant tumor progression was really unexpected and misled every possible medical interpretation, leading to rapid worsening of the patient's clinical conditions and no chance for chemotherapy treatment. The tumor mimicked the picture observed in the acute leukemia, developing diffuse infiltration in all serosal membranes, liver, lung, kidneys, multiple lymph nodes, and meninges, as revealed by the post-mortem medical report.