BACKGROUND: In Hong Kong, people aged 50-64 years were added as a recommended priority group (recommended group) for influenza vaccination by the Department of Health (DH) starting from 2011/12 onwards. The coverage rate of influenza vaccination for this age group was suboptimal at 8.5 % in 2012/13. This study investigates the factors associated with the uptake of influenza vaccination among adults in Hong Kong aged 50-64 years. METHODS: A case-control study was conducted in communities by street intercept interviews from 17 July to 15 August 2013. Cases were adults aged 50-64 years who had received the influenza vaccine in 2011/12 or 2012/13, while controls were the same as the cases, except they had not received the influenza vaccine in 2011/12 or 2012/13. Multiple logistic regression analysis was performed on the data to explore the associations between vaccination status and the variables. RESULTS: Six hundred and four respondents in total were interviewed and included in the analysis. There were 193 cases (vaccinated) and 411 controls (non-vaccinated), with a case-to-control ratio of 1:2.1. The following were strongly associated with vaccination compared to other factors: 'eligible for free government vaccine' (OR6.38, 95 % CI, 3.43-11.87, p < 0.001); 'willing to receive flu vaccination for free' (OR4.84, 95 % CI, 2.13-11.03, p < 0.001); 'perceived having severe or moderate symptoms when contracting flu' (OR2.90, 95 % CI, 1.21-6.97, p = 0.02), and 'convenient to reach a vaccination location' (OR2.87, 95 % CI, 1.06-7.74, p = 0.04). The majority of the cases (80.8 %) and controls (93.9 %) were not aware that they belonged to a recommended group for influenza vaccination and most (>80 %) were willing to be vaccinated if it was free. CONCLUSIONS: Factors related to free and convenient vaccination, the perception of the severity of symptoms when contracting influenza had a comparatively strong association with influenza vaccination uptake amongst 50-64 year olds, compared to other factors.