To develop psychometric measures specific to the ambiguities encountered in medicine and determine their value in predicting medical students' attitudes towards patients and their choice of residency, we administered to senior and first-year medical students a 25-item Likert-type questionnaire to assess their intolerance of ambiguity (ITA). Factor analysis yielded two dimensions that were converted to scales: 'Aversion to uncertainties in clinical medicine' (ITA1) and 'Preference for highly structured training environs' (ITA2). First-year students scored higher on ITA1 and lower on ITA2 than seniors. An excessive reliance on high-technology medicine, a negative orientation toward psychological problems, and Machiavellianism predicted ITA1. ITA1 was the best predictor of senior medical students' negative attributional style toward hypochondriac, geriatric and chronic pain patients. The following rank order of seniors' career choice was predicted by ITA1 scores: internal medicine, psychiatry and family medicine (lowest); radiology, surgery and anaesthesiology (highest). And by ITA2 scores: surgery, obstetrics and gynaecology, and surgical subspecialties (lowest); radiology, psychiatry and anaesthesiology (highest). We concluded that personality traits and role characteristics which predict 'Aversion to uncertainties in clinical medicine' are maladaptive to managing many primary care patients, and this mismatch is reflected in seniors' residency choice.