A 34-year-old female with left-sided ulcerative colitis (UC) developed severe febrile neutropenia/thrombocytopenia soon after infliximab induction therapy. There was no other plausible cause of the cell line abnormalities other than an accurate temporal association with infliximab administration. Supportive care, broad-spectrum antibiotic, and single dose of filgrastim was given on day 5 of admission due to persistently low absolute neutrophil count and fevers. The cell lines recovered, fever resolved and the patient made a complete clinical recovery. Clinicians should be aware of this potential life-threatening inflixmab side effect previously unreported in adults with UC.