The most important aim of diabetes education is to alter the self-care behaviour of patients with diabetes. In order to change their behaviour its determinants must be known. The pretest of a multicentre evaluation study with 558 participating insulin treated patients with diabetes was analysed to test the usefulness of the attitude-behaviour theory of Fishbein & Ajzen in explaining and possibly changing diabetes related active self-care behaviour. The theory of Fishbein & Ajzen is based on the assumption that human behaviour is reasoned behaviour. The theory views a person's intention as the immediate determinant of action. Determinants of intention are attitude and social norm. The results showed that the attitude was the most important determinant of active self-care, while a sufficient level of knowledge and a low orientation on the powerful others health locus of control scale were prerequisites for a positive attitude. The influence of the social environment was detrimental; although people tried to motivate patients to active self-care, they could not provide any real help in performing this desired behaviour. According to the results of this study, diabetes education should first aim at improving the level of knowledge and the health locus of control of the patients and second, at a positive attitude to active self-care. It is necessary to educate the social environment to create a more supportive atmosphere for the patient with diabetes.