AIM: To identify the beliefs and attitudes of a sample of obese and overweight Spanish women undertaking a weight-loss treatment. BACKGROUND: Obesity is a global epidemic. Weight-loss treatments focus on changing eating behaviours; however, many patients fail to adhere to the diet. This suggests that more effective behaviour-change interventions are required to help people change their eating behaviours. According to the theory of planned behaviour (TPB) human behaviour is influenced by beliefs. Identification of people's beliefs is an essential step in the design of behaviour-change interventions. DESIGN: A qualitative approach was employed using semi-structured interviews to interview participants. Seventeen obese and overweight Spanish women enrolled in a weight-loss treatment were recruited using a criterion sample strategy. FINDINGS: Participants' reported beliefs referring to the benefits of losing weight and the emotions related to dieting. The more positive these beliefs were the more positive their attitude towards the diet seemed to be. Findings highlight the importance of follow-ups in creating a subjective norm to maintain a diet. Perceived behavioural control to diet seemed to be related to beliefs about social support; whereas beliefs about lack of willpower to overcome temptations seemed to decrease perceived control over eating behaviours. CONCLUSIONS: Participants reported several beliefs and attitudes that offer plausible explanations of their eating behaviours. Data from this study seemed to fit the propositions of the TPB and could be used in further research to develop effective eating behaviour-change interventions. RELEVANCE TO CLINICAL PRACTICE: Nurses could use the TPB to assess individuals' attitudes, beliefs and expectations when following a diet. That assessment would provide insight into what aspects are relevant for individual patients when dieting, which could lead to more effective diet behaviour-change interventions being designed.