Individuals with neurofibromatosis type 1 (NF1) have an increased risk of developing benign and malignant tumours. The NF1 gene is thought to be a tumour suppressor gene, yet no direct proof at the molecular level exists to support this hypothesis. Here we describe a neurofibrosarcoma from a patient with NF1 with loss of heterozygosity for all chromosome 17 polymorphisms tested. On the remaining chromosome 17 homologue, a 200 kilobase (kb) tumour specific deletion of NF1 was demonstrated. This is the first example of a homozygous inactivation of NF1 at the molecular level in a malignant tumour from an NF1 patient and the results strongly support the tumour suppressor gene hypothesis for this disease.