Ataxia telangiectasia (A-T) is a rare neurodegenerative immunodeficiency syndrome, characterised eponymously by progressive cerebellar ataxia and telangiectatic vessels, most often cutaneous. A-T displays autosomal recessive inheritance, is characterised by defective DNA repair due to mutations within the ATM gene on chromosome 11 [ 1 ]; and a lack of native ATM protein gives rise to the aforementioned hallmarks of the disease. With A-T comes a predisposition to haematological malignancy and humoral immunodeficiency, and as such these patients often suffer truncated lifespans [ 1 , 2 ]. Urological manifestations are rare, and intravesical involvement comprising bladder telangiectasia has only been described in a handful of cases, all of which have previously been treated with cyclophosphamide for malignancies, a known instigator of bladder haemorrhage [ 345 ]. Similarly, BK polyomavirus is a known precipitant of nephropathy and haemorrhagic cystitis in actively immunosuppressed renal transplant patients, though has not been established as such in genetically immunodeficient patients with native urological tissue [ 6 ]. Here we present, to our knowledge, the first case of refractory haemorrhagic cystitis in an A-T patient with intercurrent BK polyoma virus infection, who had previously been treated with cyclophosphamide for leukaemia, and propose that recurrent cystoscopic diathermy is a robust and relatively conservative surgical option to treat haemorrhagic cystitis secondary to bladder wall telangiectasia.