The association between antibiotics and risk of cancer has been addressed in different studies, most of which were addressing breast cancer. The objective of this study was to assess the association between antibiotics use and risk of prostate cancer. We carried out a population-based case-control study using data from Saskatchewan Health administrative databases (Canada) between the years 1981 and 2000. Cases identified by the Saskatchewan Cancer Agency were matched to 4 controls, using incidence density sampling. The effect of dosage and timing of antibiotic use, over a minimum of 15 years before diagnosis, on prostate cancer risk was assessed. Number of prescriptions and number of tablets were used as exposure definitions. Moreover, the effect of different classes of antibiotics on prostate cancer was also studied. A total of 4,052 prostate cancer cases and 16,208 matched controls were included in this study. Antibiotics exposure (number of prescriptions) during the period of 1-15 years in the past was significantly associated with an increased risk of prostate cancer; RR = 1.69, 2.61, 2.71, and 2.83 for the 4 quartiles, respectively, p-trend = 0.0001. When number of units was taken as the exposure definition, similar results were found. We did not find any effect of the timing or class of antibiotic exposure on prostate cancer risk. We found a dose-dependent association between antibiotics exposure up to 15 years in the past and risk of prostate cancer. However, the lack of temporal trends and the absence of class specific effects suggest a noncausal relationship.