I sold some mutual funds last year. DoI have to put in cost basis?

[VolvoGirl]

Yes unless you import the sale info and it imports the cost too.  Otherwise you will be paying tax on the whole sales price.  You have to check it over close when you import a 1099B.  Also if you have any reinvested dividends you get to add those back into the cost.  They bought more shares.  It can be tricky to figure out the cost especially if you didn't sell all the shares.  And you might have to split each sale into 2 sales  Long Term and Short Term.  Any dividends reinvested or shares bought during the prior 12 months will be Short Term.   Purchased over 12 months before are long term sales.