Varicella is a common, usually self-limiting, contagious childhood disease. Its clinical presentation in generally healthy children is characteristic enough to allow one to make a diagnosis; however, in immunosuppressed patients the diagnosis may be difficult due to atypical course of the disease. This article presents a diagnostically challenging case of a 5-year-old boy with newly diagnosed acute lymphoblastic leukaemia who developed varicella-zoster infection during induction steroid therapy, with an atypical, mild course without fever or pruritus, with eruptions of changed morphology. The presented case shows that in children with leukaemia who have been in contact with individuals with varicella, the occurrence of any rash should raise suspicion of the disease. In this group of patients the diagnostic method of choice is PCR test of skin eruption material. Intravenous acyclovir therapy should be started already before the PCR result comes back.