Box 20 on a T5008 form is empty. The shares were purchased more than 20 years ago and neither I nor the bank knows the cost. I should have a capital loss. What can I do?

Most likely since the company went bankrupt the shares are most likely worthless. Owners of common shares often get nothing when a company enters liquidation since they are the last ones for payment. 

The shares will be deemed to have been disposed of for proceeds of nil at the end of the year and to have been reacquired for an adjusted cost base (ACB) of nil immediately after the end of the year. As a result, the taxpayer will be able to realize the capital loss on the property.

For more information please refer to the following Canada Revenue Agency article:

• Income Tax Folio S4-F8-C1, Business Investment Losses

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