Objective All cancer sites show considerable social inequality in risk and patient survival. Despite the advantages of healthcare professionals (HPs) systematically registering information about patients’ social circumstances, this is not routine practice. Our aim was to understand the barriers to registering patients’ social histories. Methods We observed 104 outpatient encounters and interviewed 30 HPs treating malignant melanomas, urological cancer or lung cancer. We explored interactive (e.g. processes and activities) and interpretative (e.g. perspectives and meanings) aspects of barriers to recording patients’ social data. Using Kirk´s theoretical framework developed in hospital settings, we conducted a qualitative analysis to address organizational and professional barriers. Results Unevenness in registration was due to 1) organizational barriers (interdisciplinarity, time and technical challenges), 2) professional barriers (prioritization, meeting patients where they are, and reliance on HP’s own judgements). HPs neglected to use medical records, preferring to form their own pictures of patients´ needs to avoid prejudices and to promote equality. However, HPs did not systematically verify these impressions with the patients. Conclusion To facilitate systematic recording of patients’ social histories and address the barriers identified in our study, there is a need to: 1) develop a electronical tool for recording patients’ social circumstances that is aligned with HP’s daily practices 2) introduce training programmes targeting all barriers, and 3) provide time and opportunities to support patients.