We identify the dietary factors, particularly fat intake, associated with advanced prostate cancer. We performed a case series analysis restricted to prostate cancer cases recruited in a hospital based, case control study evaluating the influence of diet on the development of prostate cancer. Patients eligible for study inclusion were 45 years old or older with prostate cancer who had been hospitalized for initial surgical treatment or radiotherapy between October 1990 and December 1992 at 1 of 8 hospitals in the Quebec City area. A diet history questionnaire was administered to 427 patients with prostate cancer of whom 384 (90%) had completed a reliable questionnaire, including 142 with advanced and 242 with local stages I and II disease. Logistic regression analysis was used to evaluate the associations between dietary intake and advanced prostate cancer. Odds ratios and trend odds ratios were adjusted for patient age, energy intake, method of cancer detection and cancer treatment. There was a moderate, not statistically significant relation to total fat (trend odds ratio 1.15, confidence interval [CI] 0.95 to 1.39). Cases in the highest quartile of saturated fat consumption had a statistically significant odds ratio of 2.15 (CI 1.14 to 4.04). In addition, the relation increased proportionally and significantly with saturated fat intake (trend odds ratio 1.24, CI 1.02 to 1.51). Inverse associations of borderline significance were observed between advanced cancer, and polyunsaturated fat (trend odds ratio 0.88, CI 0.73 to 1.07) and linoleic acid intake (trend odds ratio 0.86, CI 0.71 to 1.04). A positive trend was observed for total animal fat intake (trend odds ratio 1.20, CI 0.99 to 1.46), while a negative trend was noted for total vegetable fat intake (trend odds ratio 0.84, CI 0.70 to 1.01). This study suggests an association between saturated fat consumption and prostate cancer progression. If cohort studies performed on patients with local prostate cancer confirm these results, dietary fat intake modification may be a promising intervention to prevent prostate cancer progression.