Background: The association between dietary factors and the occurrence of prostate cancer has been studied extensively, but there is, as yet, no published study on the relationship between diet and disease progression among prostate cancer patients. We studied the association between dietary fat intake and prostate cancer survival. Methods: We prospectively followed 384 men diagnosed with prostate cancer between 1990 and 1992 in the Quebec City area who participated in a case-control study of diet in relation to prostate cancer occurrence. Trained nutritionists interviewed the men on their usual diet using a diet history questionnaire. Deaths in the follow-up were documented through record linkage with the provincial mortality file and review of hospital records. The cause of death was taken as written on the death certificate. Cox proportional hazards models were used to estimate the relative risk of dying from prostate cancer associated with terciles of fat intake, expressed as percent of dietary energy, while controlling for prognostic factors and total energy. Results: The median duration of follow-up was 5.2 years. During the follow-up period, 32 patients died of prostate cancer and 39 died of other causes. The 5-year disease-specific survival was 91%. After controlling for grade, clinical stage, initial treatment, age and total energy intake, we found that saturated fat consumption was significantly associated with disease-specific survival (p = 0.008). Compared to men in the lower tercile of saturated fat, those in the upper tercile had three times the risk of dying from prostate cancer (hazards ratio 3.13, 95% confidence interval 1.28–7.67). Conclusion: Our findings suggest that, if saturated fat is causally related to disease-specific survival, a moderate reduction of its intake below 10% of energy should reduce the risk of dying from prostate cancer. This dietary goal is already recommended for health promotion and primary prevention of heart disease and cancer.