The US military has started accepting gay recruits after a California judge last week struck down the "don't ask, don't tell" policy barring openly gay people from serving.
But the US Department of Defense says new gay recruits are warned the repeal of the law may be overturned.
The Pentagon is appealing against the decision and had asked the judge to reinstate the ban in the interim.
But Judge Virginia Phillips on Tuesday formally refused that request.
Some gay activist groups were planning to send people to enlist at recruiting stations to test the Pentagon's announcement that it was accepting recruits who openly state that they are gay.
"If they were to self admit that they are gay and want to enlist, we will process them for enlistment, but will tell them that the legal situation could change," Douglas Smith, spokesman for US Army Recruiting Command at Fort Knox, Kentucky, said.
Democrats in the US Senate had attempted to overturn the "don't ask, don't tell" policy in September, but failed to muster the necessary votes.
President Barack Obama has vowed to end the policy, but most advisers agree the president cannot end the ban on gays serving openly in the military without congressional or legal action.
Meanwhile, the Pentagon is due to release a report about the possible impact of allowing openly gay service-members on 1 December.
Some Pentagon officials have said allowing openly gay military personnel would necessitate dramatic policy changes on everything from housing and insurance to protocol at social events.
In California, Judge Phillips, declared that the policy violated gay military members' rights to free speech and to equal protection under the law.
The lawsuit that prompted the injunction against the ban was brought by the Log Cabin Republicans, a pro-gay Republican group, on behalf of openly gay military personnel who had been discharged.