It is just as important to report accurate capital losses to CRA as it is to report accurate capital gains.  You can go back and apply losses up to three years if you had capital gains in those years. 

Usually you would carry the loss back to the earliest of the three years so you don’t lose the opportunity before it drops out of the three year range.  Capital losses from stocks can be carried forward indefinitely.

This information is provided as an information service only and is not intended to substitute for competent professional advice. No action should be initiated without consulting a professional advisor.