BK polyomavirus-associated haemorrhagic cystitis (BKHC) is a complication after allogeneic stem cell transplantation, which can occur in 5–60% of the cases. BK viruria alone can also occur in up to 100%. BKHC can lead to severe morbidity in stem cell-transplanted patients, but data about this disease is limited. Consequently, we conducted a prospective unicentric non-interventional trial on BKHC as well as BK viruria after first adult allogeneic stem cell transplantation with a follow-up time of 1 year after inpatient treatment. Between November 2013 and December 2015, we were able to include 40 adult patients with a mean age of 52.8 years. Twenty-seven (67.5%) of these patients were male and 13 (32.5%) were female. Acute myeloid leukaemia was the most frequent underlying disease (n = 15; 37.5%). Only 1 patient developed BKHC during inpatient treatment (n = 1; 2.5%), but BK viruria was frequent (n = 11; 27.5%) during inpatient treatment as well as in the follow-up time (n = 14; 35%). Interestingly, BK viruria was significantly associated with mucositis (p = 0.038) and number of transfused platelet concentrates (p = 0.001). This unexpected association will be discussed and needs further investigation.