In approximately 20% of patients with suspected allergies, no organic symptom explanation can be found. Limited knowledge about patients with "medically unexplained symptoms (MUS)" contributes to them being perceived as "difficult" and being treated inadequately. This study examined the psychobehavioural characteristics of patients presenting for a diagnostic allergy work-up. Patients were interviewed and completed various self-rating questionnaires. Patient-Doctor interaction was evaluated, and the organic explicability of the patients' symptoms was rated by allergists. Patients with vs. those without MUS differed in several respects. Mental comorbidity, female sex, dissatisfaction with care, and a problematic countertransference (the interviewer's feelings towards the patient) independently predicted MUS. Patients whose symptoms could be explained organically reported more psychobehavioural problems than a control group of immuno-therapy patients. There were no differences in patient-doctor interaction. In patients with suspected allergies, recognition of psychological burden and concurrent mental disorders is important. Mental comorbidity and a difficult patient-doctor interaction may predict MUS.