BACKGROUND: Hot weather leads to increased illness and deaths. The Heatwave Plan for England (HWP) aims to protect the population by raising awareness of the dangers of hot weather, especially for those most vulnerable. Individuals at increased risk to the effects of heat include older adults, particularly 75+, and those with specific chronic conditions, such as diabetes, respiratory and heart conditions. The HWP recommends specific protective actions which relate to five heat-health alert levels (levels 0-4). This study examines the attitudes to hot weather of adults in England, and the protective measures taken during a heatwave. METHODS: As part of a wider evaluation of the implementation and effects of the HWP, a survey (n = 3153) and focus groups, a form of group interview facilitated by a researcher, were carried out after the June 2017 level 3 heat-health alert. Survey respondents were categorised into three groups based on their age and health status: 'vulnerable' (aged 75+), 'potentially vulnerable' (aged 18-74 in poor health) and 'not vulnerable' (rest of the adult population) to hot weather. Multivariable logistic regression models identified factors associated with these groups taking protective measures. In-person group discussion, focused on heat-health, were carried out with 25 people, mostly aged 75 + . RESULTS: Most vulnerable and potentially vulnerable adults do not consider themselves at risk of hot weather and are unaware of the effectiveness of important protective behaviours. Only one-quarter of (potentially) vulnerable adults reported changing their behaviour as a result of hearing hot weather-related health advice during the level 3 alert period. Focus group findings showed many vulnerable adults were more concerned about the effects of the sun's ultra-violet radiation on the skin than on the effects of hot temperatures on health. CONCLUSIONS: Current public health messages appear to be insufficient, given the low level of (potentially) vulnerable adults changing their behaviour during hot weather. In the context of increasingly warmer summers in England due to climate change, public health messaging needs to convince (potentially) vulnerable adults of all the risks of hot weather (not just effects of sunlight on the skin) and of the importance of heat protective measures.