Staff at the Walter Reed National Medical Centre were ordered to shelter in place until police with sniffer dogs had finished inspecting its 20-story main building.
"At this time, there's been no injuries reported (and) no indication, no evidence of any kind ... That there was a shooting," Montgomery County Police spokesman Captain Paul Starks told reporters.
Officers have interviewed the individual who made the initial call at 10:48 am (1448 GMT), he said, without giving further details.
Staff took cover as a "code white" lockdown was declared at the sprawling facility where 7,000 people are employed and security is described as tight.
One member of staff described colleagues and patients hiding in a bathroom with the lights off, amid shouts in the hallways for people to stay put.
Local TV news meanwhile showed hospital staff -- some in white smocks -- exiting the main building with their hands in the air.
The lockdown at Walter Reed -- the nation's premier military hospital -- followed a weekend of heightened security in the United States for the Independence Day holiday.
The Navy Yard was the scene of a mass shooting in September 2013 in which a lone gunman killed 12 people and injured three others.
Walter Reed is regarded as the top US military hospital, with patients ranging from Iraq war veterans to President Barack Obama.
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