BACKGROUND Bacillus Calmette-Guérin (BCG) is considered the most effective treatment to reduce recurrence and progression of non-muscle invasive bladder cancer (NMIBC) but can induce local side effects leading to treatment discontinuation or interruption. Aim of this exploratory study is to investigate if the sequential administration of Hyaluronic acid (HA) may reduce local side effects of BCG. METHODS 30 consecutive subjects undergoing BCG intravesical administration for high risk NMIBC were randomized to receive BCG only (Group A) or BCG and HA (Group B). A 1 to 10 Visual Analog Scale (VAS) for bladder pain, International Prostate Symptom Score (IPSS) and number of micturitions per day were evaluated in the two groups before and after six weekly BCG instillations. Patients were also evaluated at 3 and 6 months by means of cystostopy and urine cytology. RESULTS One out of 30 (3,3%) patients in group A dropped out from the protocol, for local side effects. Mean VAS for pain was significantly lower in group B after BCG treatment (4.2 vs. 5.8, p = 0.04). Post vs. pre treatment differences in VAS for pain, IPSS and number of daily micturitions were all significantly lower in group B. Three patients in group A and 4 in group B presented with recurrent pathology at 6 month follow up. CONCLUSIONS These preliminary data suggest a possible role of HA in reducing BCG local side effects and could be used to design larger randomized controlled trials, assessing safety and efficacy of sequential BCG and HA administration. TRIAL REGISTRATION NCT02207608 (ClinicalTrials.gov) 01/08/2014. Policlinico Tor Vergata Ethics Committee, resolution n 69-2011.