Early identification and support of strugglers in medical education is generally recommended in the research literature, though very little evidence of the diagnostic qualities of early teacher judgments in medical education currently exists. The aim of this study was to examine the validity of early diagnosis of struggling in medical school based on informal teacher judgements of in-class behavior. The study design was a prospective cohort study and the outcomes/truth criteria were anatomy failure and medical school drop out. Six weeks into an anatomy course, student tutors attempted to identify medical students, who they reckoned would fail the anatomy course or drop out, based on their everyday experiences with students in a large group educational setting. In addition, they were asked to describe the indicators of struggling they observed. Sixteen student tutors evaluated 429 medical students for signs of struggling. By week six, the student tutors were able to detect approximately 1/4-1/3 of the students who eventually failed or dropped out, and for ¾ of the strugglers they identified, they were correct in their judgments. Informal student tutor's judgements showed incremental validity for both outcomes when controlling for grades obtained in preceeding exams. Lack of participation, lack of commitment, poor academic performance, poor social interactions and general signs of distress were the main indicators of struggling identified. Teachers' informal judgements of in-class behavior may be an untapped source of information in the early identification of struggling medical students with added value above and beyond formal testing.