Immune reconstitution inflammatory syndrome (IRIS) occurs when a patient is recovering from a transient immunodeficiency and results in an uncontrolled inflammatory response to infectious agents and tissue damage. Symptoms such as fever and radiological signs seem to paradoxically appear or worsen, unmasking a previously unrecognized infection. The patient's clinical condition may then deteriorate as a result of increasing tissue damage and this may even lead to death. IRIS was initially described in patients suffering from a HIV infection who experienced immune recovery following the initiation of antiretroviral therapy. Increasingly, however, the syndrome is being reported in patients who are recovering from an episode of neutropenia following chemotherapy, hypomethylating agent use or a stem cell transplantation for the treatment of a solid tumour or haematological cancers. We describe two cases of IRIS following an episode of neutropenia in patients with a haematological malignancy and elaborate on the pathogenesis, diagnosis and treatment of IRIS in cancer patients.