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Received mysterious K-1 form, seeking answers
SXL is a Master Limited Partnership so all of the income is pass-through.  Your equity purchase entitles you to a fraction of the 66% of the company that is not owned by Energy Transfer Partners.  You should have been receiving the K-1s from SXL from the time that you bought the shares.   Without knowing your specific situation, you will likely have to amend your returns for at most 6 years (if the omitted amount of gross income exceeds 25% of your gross income originally stated as littleadv has graciously pointed out in the comments) and include Schedule E to report the additional income (you'll also be able to deduct any depreciation, losses etc. that are passed through the entity on that form, so that will offset some of the gains).  As littleadv has recommended, speak with a tax professional (CPA/EA or attorney) before you take any further steps, as everyone's situation is a bit different. This Forbes article has a nice overview of the MLP.  There's a click-through to get to it, but it's not paywalled.