We examined coded emotional expression during an initial therapy session and its association with a known physiological risk factor for early death, aberrant diurnal cortisol slope, in women with metastatic breast cancer. Out of 64 women with metastatic breast cancer randomized to a multi-site clinical intervention trial of supportive-expressive group therapy (SET), a subsample of 29 met eligibility criteria for this study. We tested whether longer mean durations of primary negative affect (fear, sadness, and anger) expression were associated with steeper diurnal cortisol slopes after adjusting for speaking time, repressive-defensiveness, anxiety, and the interaction between repressive-defensiveness and anxiety. We found that steeper cortisol slopes were related to lower repressive-defensiveness and greater primary negative affect expression in line with a priori hypotheses. Additionally we explored whether coded positive affect, defensive/hostile affect, constrained anger, and the interaction between primary negative affect and repressive-defensiveness explained additional variance in diurnal cortisol patterns.