Pyoderma gangrenosum is a poorly understood, ulcerating cutaneous disorder which is rarely seen in the paediatric age-group. We report a 3-year-old boy who developed an ulcer over the left frontoparietal scalp at the age of 1 year. A 9-cm area of underlying cranial bone was destroyed. The appearance on radiographs and CT scan was suggestive of eosinophilic granuloma, osteomyelitis, or other destructive processes. Biopsies of the scalp lesion and calvaria showed granulation tissue and degenerating bone. After the biopsies the scalp lesion increased in size, and wound dehiscence occurred. Ulceration developed at the site of a PPD skin test, which on biopsy was consistent with the diagnosis of pyoderma gangrenosum. Pyoderma gangrenosum should be added to the differential diagnosis of cutaneous disorders which can result in osteolytic/osteonecrotic defects.