mlp can be held in a retirement account; there is one way you theoretically could deduct a loss in your 401(k). If you made nondeductible (or after-tax) contributions to your 401(k) and your losses were so large that the amount you withdrew from your account was less than the amount you contributed with after-tax dollars, then you could take a deduction. For that to happen, however, the value of all the pre-tax dollars you contributed, plus any match your employer made, would have had to fall zero, essentially leaving you only with whatever remained of your after-tax contributions. In addition, many IRAs and 401(k)s are funded with money that isn't included in your taxable income. If yours is one of them, you might not be able to claim a deduction at all.