BACKGROUND The aim of this study was to assess the associations of comorbidities with primary treatment of prostate cancer (PCa) patients and of comorbidities with PCa-specific mortality (PCSM) compared to other-cause mortality (OCM) in Switzerland. PATIENTS AND METHODS We included 1527 men diagnosed with PCa in 2000 and 2001 in the canton of Zurich. Multiple imputation methods were applied to missing data for stage, grade and comorbidities. Multinomial logistic regression analyses were used to explore the associations of comorbidities with treatment. Cox regression models were used to estimate all-cause mortality, and Fine and Gray competing risk regression models to estimate sub-distribution hazard ratios for the outcomes PCSM and OCM. RESULTS Increasing age was associated with a decreasing probability of receiving curative treatment, whereas an increasing Charlson Comorbidity Index (CCI) did not influence the treatment decision as strongly as age. The probability of OCM was higher for patients with comorbidities compared to those without comorbidities [CCI 1: hazard ratio 2.07 (95% confidence interval 1.51-2.85), CCI 2+: 2.34 (1.59-3.44)]; this was not observed for PCSM [CCI 1: 0.79 (0.50-1.23), CCI 2+: 0.97 (0.59-1.59)]. In addition, comorbidities had a greater impact on the patients' mortality than age. CONCLUSIONS The results of the current study suggest that chronological age is a stronger predictor of treatment choices than comorbidities, although comorbidities have a larger influence on patients' mortality. Hence, inclusion of comorbidities in treatment choices may provide more appropriate treatment for PCa patients to counteract over- or undertreatment.