Anaplastic large cell lymphoma (ALCL) is a rare non-Hodgkin's lymphoma, with rearrangements of the anaplastic lymphoma kinase gene in 60%-85% of systemic cases. We report an 11-year-old boy with ALCL in whom serial FDG PET/CT revealed partial response and complete metabolic response at interim and end of treatment, respectively. However, the patient relapsed within 2 weeks, confirmed by cytology. Because of the inherent aggressive nature of ALCL, possibility of an early relapse should always be kept in mind.