This is humira at work. Next here tonight, the news about polygamy and whether a man can live with more than with one wife, to be clear it's absolutely illegal to have a marriage license with more... See More
This is humira at work. Next here tonight, the news about polygamy and whether a man can live with more than with one wife, to be clear it's absolutely illegal to have a marriage license with more than one person at one time. But a big court decision coming out of Utah seems to say it's okay if you just skip the official piece of paper. ABC's Cecilia Vega shows us the issue that has everyone arguing. Reporter: One husband, four wives, 17 children. We know them as reality stars on the hit show, "Sister wives." I need flowers for my wives. Reporter: Today, they are unlikely legal crusaders in a fight over the fate of polygamy. I think that anybody should be able to organize their family according to how they choose. Reporter: It is an epic court battle -- Kody brown and his wives against the state of Utah, they maintain theirs is a healthy lifestyle. But that is not how authorities in Utah see it, threatening the browns with prosecution after they flaunted their plural family on TV. I just want to be a free man. Reporter: A free man -- but what about the women and children? One study concluded polygamy leads to "Greater levels of crime, child neglect and abuse." The browns say those famous images of convicted pedophile Warren Jeffs and the flds --that's not their family or any other polygamists they know. And in December, a federal judge sided with them, declaring a key part of Utah's polygamy law unconstitutional. Co-habitation is legal, so Kody brown can openly live with all of his wives. Thousands of people living in plural marriage in Utah are now free. Reporter: But opponents say there could be unintended consequences. Could this ruling that helps your family also be helping believers and followers of Warren Jeffs? I guess what we're hoping -- Reporter: This lawyer says the ruling actually makes it tougher for authorities to investigate potential abuse. It's very sad that we have a Hollywood polygamist family that has made it harder to protect the women and children in most polygamist families. Reporter: The browns plan to stay in Nevada. Does this feel like home now? These reality stars say they're prepared to go all the way to the supreme court. Cecilia, Vega, ABC news, las Vegas.
This transcript has been automatically generated and may not be 100% accurate.