Unilateral laterothoracic exanthem (ULE) developed in a 4-year-old-boy on maintenance therapy for acute lymphatic leukemia (ALL). This is the second reported case of an association between ULE and leukemia. The etiology of ULE is unknown. Endemic cases, frequently associated signs of upper respiratory infections, and the occurrence of ULE in immunocompromised hosts are suggestive of an infectious process.