Objective: Sound levels in nightclubs are dangerously high. We administered the Dangerous Decibels hearing-health intervention to nightclub staff to test its efficacy. Design: In a single group, repeated measures were taken before training, a week after training and at 3 months after training. Setting: A nightclub in the city of Auckland, New Zealand. Method: We delivered training to 20 people who work in nightclubs: bar staff, disc jockeys (DJs), security staff and police. We assessed supports and barriers towards hearing-health behaviour, knowledge, attitudes and self-reported behaviour at the three time points. Results: The ratio of supports to barriers for good hearing-health behaviour improved 1 week after training and continued to improve at 3 months. Participants' knowledge increased after training and was maintained 3 months later. Attitudes and self-reported behaviour did not change. Conclusion: The continued improvement in supports-to-barriers ratio at 3 months post-training has not been observed previously and may reflect a change in participants' thinking as a result of the intervention. The lack of change in self-reported behaviour implies that the effect of acculturation to loud music in nightclubs was not wholly overcome.