no, you don't have to report the cost. but yes you have to report the CIL because proceeds are reported to the iRS. the effect on your return could be minor depending on what the cost would be. CIL usually results when there is a stock split, reorg , merger, etc and the company would need to issue fractional shares. rather than do that the frac share is sold at market value and you receive the proceeds. in theory, the cost of the original shares should be allocated among the new share including the fractional shares. many don't do this because either they don't know to do this or the cost allocated is small. technically if they were to sell the stock, the cost basis would be overstated because cost wasn't allocated to the frac shares.
