Background: Surgeons in training undertake procedure-based assessment (PBA) with their trainers. Time pressures limit when this occurs. Self-assessment would provide additional opportunities but requires evaluation of your own skills and knowledge. Aim: To investigate the validity of a self-assessment PBA in the operating theatre and evaluate learning needs and change in practice identified. Methods: In a prospective study, postgraduate UK trainees performed a self-assessment PBA after formally reflecting on appendicectomy surgery. Later, they performed their usual external assessment PBA on the same case and outcomes were compared. Learning outcomes were analysed independently by two trainers. Results: There were 25 trainees (14 males), of age 29 years (25-33). They had performed a median of five previous appendicectomies (2-21) after a median three years of speciality training (range 1-4). There were no significant differences in global summary scores, trainee satisfaction or learning outcomes between external and self-assessment PBAs. Construct validity of the self-assessment PBA was demonstrated. The self-assessment PBA was more likely to identify non-technical skills. Conclusion: Our results suggest self-assessment PBA is valid for formative assessment. It identified a wide range of learning outcomes. Self-assessment PBA would help trainees maximise every learning opportunity and practice self-assessment skills.