Share your insights or perspective on the financial matter presented in the input.

Is it smart to only invest in mid- and small-cap stock equity funds in my 401(k)?
The benefit of the 401K and IRAs are that reallocating and re balancing are easy. They don't want you to move the funds every day, but you are not locked in to your current allocations. The fact that you mentioned in a comment that you also have a Roth IRA means that you should look at all retirements as a whole.  Look at what options you have in the 401K and also what options you have with the IRA. Then determine the overall allocation between bonds, stocks, international, REIT, etc. Then use the mix of funds  in the IRA and 401K to meet that goal. Asking if the 401K should be small and mid cap only can't be answered without knowing not just your risk tolerances but the total money in the 401K and IRA.  Pick an allocation, map the available funds to that allocation. Rebalance every year. But review the allocation in a few years or after a life event such as: change of job, getting married, having kids, or buying a house.