This study investigated prospectively the relationship between optimism, threat appraisal, seeking support and information, cognitive avoidance, physical treatment side effects, and decision-related distress in 111 men with localized prostate cancer. Men were assessed at diagnosis and 2 and 12 months after treatment. Baseline decision-related distress predicted distress 2 and 12 months after treatment. Optimism was a significant prospective and concurrent predictor of decision-related distress, with the effect mediated by proximal cancer threat appraisal. Seeking support and information and cognitive avoidance were not associated with decision-related distress at any time point. For physical treatment side effects, concurrent urinary symptoms were predictive of decision-related distress 2 months after treatment. Results suggest that decision-related distress is generated by similar processes to that of the psychological distress that follows a cancer diagnosis. Screening for men with high decision-related distress for referral to in-depth decision support is suggested. Outcome expectations may present as a therapy target to increase the effectiveness of decisional support that is utility based.