Thrombosis is a common phenomenon in patients with malignancies. It is believed that thrombosis is multifactorial and that in addition to mechanisms directly associated with cancer and its treatment, it may also be related to the interaction between the immune system and clotting. The present work describes four cancer patients (three adults and one child) whose clinical course was characteristic of catastrophic antiphospholipid syndrome (CAPS) in intensive care units of the National Cancer Institute of Rio de Janeiro. The presence of findings similar to those in CAPS can be attributed to an unbalanced interaction between the immune system and coagulation.