Background: Fanconi anaemia is a rare disease associated with cellular sensitivity to chemicals (e.g. mitomycin C and diepoxybutane); variable but mild cellular radiosensitivity has also been reported.Materials and methods: A 32-year-old patient with Fanconi anaemia and tonsillar carcinoma, treated by radiotherapy, was found to exhibit profound clinical radiosensitivity. Confluent, ulcerating oropharyngeal mucositis developed after a conventionally fractionated dose of 34Gy and healing was incomplete by 2 months after cessation of therapy.Results: Cellular radiosensitivity assays and RPLD studies from this patient did not suggest any major detectable radiosensitivity.Conclusion: There is a discrepancy between the observed clinical radiosensitivity and the usual 'predictive' radiosensitivity assays in this patient with Fanconi anaemia.