This study was a case-control observational study using 25 farms that had a 2 year history of clinical Porcine Circovirus type 2 (PCV2) associated disease (PCVAD) and also 25 matched farms that had no history of the virus. A survey was taken to find the risk factors associated with the disease and samples were taken to test for what type of circovirus was present. All viruses from case or control farms were identified as RFLP 321. Vaccination has proven to be very effective in containing the disease in case farms and it was more difficult to find virus on vaccinated farms. Lesions consistent with PCVAD were found on control farms, suggesting that a vaccine might be appropriate in these herds also. The main result of this work was that the new strain of virus that prevailed 2 years ago, which appears to be a severe disease of grower-finisher pigs, is now in the majority of swine herds in Ontario. There may potentially be other reasons as to why the disease suddenly became a cause of high mortality in grow-finish pigs.