The prostate commonly develops benign prostatic hyperplasia (BPH) in dogs over 5 years, while in aged dogs other pathological findings might be revealed by ultrasonographic exam. The aim of the present study was to estimate the most suitable age for a preventive ultrasonographic examination of the prostate in the dog. The prostate of 1003 intact male dogs of 64 different breeds, of different ages (1-18 years) and bodyweights (2-55 kg) was evaluated with ultrasound, irrespective of the reason for clinical examination. The age of each dog was expressed as the ratio between the actual age and the maximum longevity expected for the breed. Dogs were divided in two groups based on breeds' life expectancy as short life (SL) and long life (LL). The size of the prostate (normal, enlarged or small) and the presence of abnormal sonographic findings were recorded for each dog. The results of the present study indicate that the most suitable age for a preventive ultrasonographic exam of the prostate in the dog is approximately at 40% of its expected longevity, both in short and long life breeds, because at this age there is a strong possibility to be able to detect abnormal prostatic findings. In 47.5% of the dogs at least one abnormal finding of the prostate was revealed by ultrasonographic exam, while dogs with long life expectancy showed a significantly higher prevalence of abnormalities, than dogs with short life expectancy. The most frequent findings were the increase of prostatic size (33.5%) and the presence of at least one cyst (33.6%), with no difference between SL and LL dogs. In conclusion, a preventive examination of the prostate starting at 40% of expected longevity in dogs of short and long life breeds is strongly recommended for early detection of abnormalities, for scheduling specific follow up and for suggesting effective therapeutic protocols.