Summary Background In contact dermatitis, it is crucial to understand and remember the outcome of patch testing. Unfortunately, many patients do not remember the results of their patch tests. Objectives Our aim was to evaluate the effects of an intervention programme in which extensive information on specific contact allergy was provided, in individuals with positive patch test reactions. Methods The study was designed as a randomized, investigator‐blinded clinical trial. Participants with positive test reactions were randomized into two groups that received either standard information according to clinical routine or standard information and a reminder letter. Knowledge of contact allergies was evaluated using questionnaires 12 months after inclusion. Results There were 184 adults included in the trial and the response rate at 1‐year follow‐up was 78% (143 of 184). Sixty‐five per cent (45 of 69) in the intervention group and 54% (40 of 74) in the control group reported the correct name of the allergen (P = 0·23). Participants with several – as opposed to few – positive patch test reactions had difficulty in remembering the correct names of the allergens (P = 0·001). Moreover, the type of allergy had an influence on their ability to remember the name of the allergen correctly and their ability to make changes in lifestyle. Conclusions The intervention performed did not significantly affect the participants’ ability to remember their contact allergy. To achieve better knowledge and changes in lifestyle, efforts to inform should concentrate on individuals with several positive patch test reactions, those with particular allergens, individuals over 60 years of age, and – concerning changes in lifestyle – males.