Houston's police chief says no one has been found injured and there is no evidence of a shooting amid reports of gunfire inside the hospital at the Texas Medical Center.
"We have not found a suspect and we have not found evidence of an active shooter," Art Acevedo told Houston TV station KHOU.
Officers made second sweep of Ben Taub Hospital, one of the city's major trauma centres, before patients and staff were allowed back inside, KHOU reported.
Police began receiving reports of gunfire shortly after 2pm (7am AEDT) on Tuesday.
Dozens of officers and SWAT team members were at the hospital.
Television images from helicopters showed dozens of employees leaving the hospital, some of them attending to patients who appeared to have been taken out on gurneys or in wheelchairs.
The hospital has nearly 500 beds and its trauma centre treats more than 100,000 emergency patients each year.
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