We report a four-year-old boy with a skin lesion that was misdiagnosed as an abscess and unsuccessfully treated with systemic antibiotics and drainage. Due to its progression, the child underwent a biopsy, which revealed myeloid sarcoma. The histopathological verification at the referral centre changed the diagnosis to an anaplastic large-cell lymphoma (ALCL) with internal organ involvement. This subtype of paediatric lymphoma usually manifests as a systemic disease, and isolated skin infiltration is rare. The patient was treated according to the ALCL-99 protocol and achieved remission. Five months later a systemic lymph-node relapse was diagnosed. Salvage chemotherapy was administered and allogeneic stem cell transplantation was performed, which resulted in sustained remission. Skin infiltrates are commonly seen in children, and routine diagnostics is usually sufficient for a proper medical care. The diagnostic difficulties in the reported patient emphasise the need for observation and invasive diagnostics in non-responding cases.