Background. Assessment of adjustment of patients in cancer treatment trials is becoming more common and increasingly regarded as a useful outcome measure. The widely used Mental Adjustment to Cancer (MAC) Scale was designed to measure Fighting Spirit (FS), Anxious Preoccupation (AP), Helpless-hopelessness (HH) and Fatalism. Methods. Questionnaire responses from 632 breast cancer patients were randomly divided into two groups, one for exploratory analyses and possible scale refinement, and the other for validation purposes. Results. Estimates of reliability (Cronbach's α) were satisfactory for two scales, FS (α = 0.85) and HH (α = 0.81), but lower for AP (α = 0.65) and Fatalism (α = 0.64). Exploratory factor analysis suggested that the MAC Scale might be measuring six independent constructs including two related to Fighting Spirit (Positive Orientation to the Illness, Minimizing the Illness), two related to Fatalism (Fatalism-revised, Loss of Control), a construct we have named Angst, and an unchanged HH construct. Scales developed to measure these constructs were satisfactorily replicated in confirmatory analyses but some reliabilities were lower than desirable. The general structure of the MAC Scale remained little changed despite the division of two scales and the suggested removal of six items. The refined scales correlated with the Hospital Anxiety and Depression scale and the Medical Coping Modes Questionnaire, indicating good concurrent validity. Conclusions. While reasonable reliability of the original scales persists through analyses of the MAC Scale, the original factor structure could not be reproduced. Six refined constructs with strong construct validity were identified within the overall domain of mental adjustment to cancer.