Is it allowed to have shares of PFIC (passive foreign investment companies) in a 401(k) account? Some PFIC are traded on the U.S. exchanges and could be easily bought through a 401(k) account.

Are the respective reporting forms (form 8621) required in this case?

I dont understand your Q. Are these investments held in your you employer's 401k. If so you should check with the plan admin. Or are these investments made under a self directed brokerage account with a broker that is permitted under the plan? Most brokerages restict trading to investments that are traded on US exchanges because holding foreign assets in a US account has regulatory problems that custodians do not want to deal with.

Well, my broker (Fidelity) allows me to buy ADRs on U.S. exchanges or OTC, and also to trade on foreign exchanges.

So the question is still unanswered. I'm not asking if it is possible (yes, it is, and I would like to diversify my retirement savings internationally), but rather if there is any legal problem. Theoretically, the answer should be either "yes" or "no" I could not find a "no" anywhere, but nobody was able to say "yes" until now. I could not find PFIC under the list of prohibited transactions. Also, do the forms have to be filled out although the account is tax-advantaged?

Thanks