Nevada has become the first US state to allow residents to play poker online for money.
Online poker was legalised in the state in February. And states across the US, probably starting with New Jersey and Delaware, look likely to follow suit.
The Nevada Gaming Control Board has issued a 30-day licence to allow websites to compete for customers, with poker site the first to go live.
Online gambling is expected to generate billions of dollars for US authorities.
Each year an estimated 10 million Americans play online poker, but the sites are currently based offshore, meaning state and federal authorities make no money from them.
Internet gambling was banned by Congress in 2006, but states across the US now hope to relax the rules.
Under the terms of the Nevada licence, players must be at least 21 years old and residents of the state. Initially, only online poker will be played.
UltimatePoker is a subsidiary of Station Casinos, which operates 16 casinos across Las Vegas.
US group Stop Predatory Gambling has said the alliance between government and the gambling industry is "actively encouraging people to lose money".
The UK is often cited as a leader in gambling regulation. Online sites are required to verify identities and link and provide support to help services for addicts.
Butt there has been a big increase in the number gambling addicts in the UK, where online betting is a £2bn industry.
In 2010, research from the Gambling Commission estimated there were 450,000 problem gamblers in the UK, with an average debt of £17,500, 200,000 more than in 2007.
Microgaming Software Systems, based in the Isle of Man, created the world's first online casino 20 years ago.