The facility began resuming normal operations after authorities conducted a search and gave an "all clear." No suspect was located.
Santa Ana police said officers received a call from the hospital around 8:15 a.m. saying there was a phone call made from an internal network phone stating that there was a man with a gun inside the hospital, said Cpl. Anthony Bertagna.
"Obviously, we came with a large police presence," the corporal said. "We're doing a systematic search. We've completed the basement and first floor -- we're working on the second and third floors. We treat this as an actual incident until we find out otherwise."
Investigators were also reviewing surveillance video in an attempt to determine where the initial phone call came from. Surgeries and other medical services continued during the lockdown, according to police and hospital officials.
There were no reports of any injuries or any shots fired.
A hospital spokesman said staff members are trained to "run, hide, fight" in a lockdown situation.
Nearby Calvary Christian School was temporarily placed on lockdown, Santa Ana police said. Tustin Avenue was reopened after being shut down between 17th and 1st streets due to the investigation.
Bertagna said police officers and hospital staff have previously trained together for such an incident.
"We've done exercises together," he said. "So everybody's doing what they need to do."
At a news conference after the incident, Police Chief David Valentin said it was unclear if the caller was a patient, visitor, employee, former employee or someone else.
"They said, 'I have a gun,' hung up the phone, essentially," Valentin said.
The police chief acknowledged the possibility that the call may have been a hoax.
"But what's most critical here ... the public, I want to reassure that when we get a call such as this, we want to sequentially respond and make sure that we render the facility safe before we make any determination to move on," he said.