The European Code against Cancer includes some primary cancer prevention behaviours, which can be studied in the framework of psychosocial models of human behaviour as the ASE model (attitude-social influence-efficacy model). The objective of this study was to detect the factors that better explain cancer behavioural risk in relatives of cancer patients. A convenience sample of 3031 people was selected in primary care centres. A three-step multivariate analysis was carried out by means of a multiple linear regression, introducing cancer behavioural risk as the dependent variable and the following covariables: psychosocial factors in the ASE model, sociodemographic variables and the family history of cancer. At least five difficulties and four needs were perceived in following the preventive advice by 25% of patients. The main difficulties were tobacco and alcohol addiction and the demands of a social life. Principal needs were access to cessation programmes, family support, and being controlled and pressured by health workers. The highest risk profile is to be a young man with a low sociocultural level. The ASE determinants were the best predictors of cancer behavioural risk, so programmes that forget these predictors may not achieve any impact and may waste resources.