Depression is common in patients with medical illness. However, little is known about frequency and clinical relevance of preoperative depression in surgical patients. The objective of this study was to investigate the frequency of depression, essential health risk factors and hospital length of stay (LOS) of patients in preoperative anaesthesiological assessment. Patients were consecutively screened in the preoperative anaesthesiological assessment clinics. In total, 5429 patients gave written informed consent to perform a computerised self-assessment of lifestyle factors, including alcohol use, tobacco smoking, weight, physical status, physical exercise, sleeping disturbance, subjective health and sense of coherence (SOC). Depression was defined by a WHO-5 well-being score of 13 or less. LOS was obtained from the electronic patient management system. A clinically relevant depressive state was found in 29.7% of the patients. Patients with depression had a median LOS of 6.0 days as compared to patients with positive well-being who had a LOS of 4.8 days (P < 0.001). Worse subjective health, less physical exercise and experience of SOC, as well as more severe sleeping disturbances were independently and significantly associated with depression (P < 0.001). Clinically significant depressive states are frequent conditions in surgical patients of preoperative anaesthesiological assessment and are associated with an increased LOS. Different clinical pathways delivering adequate preoperative information according to the needs, considering subjective health and SOC of the patient as well as avoiding immobilisation and sleep disturbances during hospital stay should be considered. Long-term treatment programmes including brief intervention in the hospital and an outpatient concept should be offered.