As part of the US Tax Reform, there is a new tax of up to 15.5% on the retained earnings of Controlled Foreign Corporations. This is a new issue, and much information on this is still rolling out, so I plan to keep you abreast on this issue.
You will wish to seek professional advice if you are an American citizen who holds at least 10% of the shares in any Canadian Corporation. You will quite likely wish to take action on this issue before you file your 2017 Corporate Tax Return to mitigate the amount of taxes you will have to pay.
You can also find some information on the issue in this article by KPMG