OBJECTIVE: To test the hypothesis that urinary catheter balloons filled with sterile water, saline or glycine have equivalent rates of failure to deflate. MATERIALS AND METHODS: This was an in vitro equivalence study designed to test whether saline or glycine are neither substantially worse nor substantially better than water in terms of balloon-deflation failure rates. Glycine was chosen as the third arm, as it is readily available during endoscopic procedures and would be useful to use in such situations. We hypothesised that balloon-deflation failure rates using saline or glycine were no worse than water by 10%. We calculated the sample size for equivalence testing; 600 catheters were randomized by computer-generated random numbers to receive 10 mL of water, saline or glycine, and then immersed in a heated artificial urine solution for 6 weeks. The catheter balloons were then deflated, noting any failures to deflate and recording the deflation volumes. RESULTS: There was no failure to deflate in all 600 catheters. The median deflation volume for water, saline and glycine was 9.0, 9.2 and 9.1 mL, respectively (P < 0.001 Kruskal-Wallis test). Post-hoc pair-wise comparisons showed that the deflation volume difference between water and saline was significant (P < 0.001), as was that between water and glycine (P < 0.001). The practical implication of this difference is not apparent from this study. CONCLUSIONS: The use of saline or glycine in catheter balloons has an equivalent deflation failure rate to using water, which in this study was zero.