A 26-year-old man undergoing therapy with 45 mg ustekinumab (Stelara) for chronic psoriasis vulgaris was referred by his general practitioner to an infectious diseases department for fatigue, fever, night sweating, generalised lymphadenomegaly and unexplained weight loss. Physical examination revealed bilateral occipital, cervical, axillary and inguinal lymphadenomegalies in addition to splenomegaly. Preliminary investigation revealed elevated Plasmodium lactate dehydrogenase and an inversion of the CD4/CD8 ratio. Whole-body spiral CT scanning with and without contrast showed splenomegaly and highlighted supradiaphragmatic and subdiaphragmatic lymphadenopathies. A complete Infectious Disease Test Panel revealed high levels of anti-Toxoplasma gondii antibodies. Immunoglobulin G avidity was negative. Peripheral blood lymphocyte phenotyping was performed to exclude underlying lymphatic neoplasia. The diagnosis of severe acute toxoplasmosis infection in the setting of immune response modifiers was made. Ustekinumab was suspended indefinitely and the patient underwent monthly serological tests to monitor the immune response until all symptoms resolved and the serological testing was negative for Toxoplasma.