The Billings Fire Department responded to a report of smoke in St. Vincent Healthcare's Fortin Pediatric Specialty Clinic shortly before 1 p.m. on Thursday.
According to the Billings Fire Department, the smoke that filled the first floor came from a mechanical room motor.
Four engines, a ladder truck and the battalion chief responded to the building at 1232 N. 30th St.
The building, which houses clinic offices with no inpatient rooms, was evacuated.
The smoke was cleared in about an hour and staff and their patients were allowed back in.