The suicide bombing occurred in Khar, the largest town and administrative center of the Bajour tribal region on the border with Afghanistan, the Dawn newspaper reported.
"The woman, in her early 20s, blew up her explosives as she walked to the main entrance of the hospital. Authorities have found the attacker's head and legs and are conducting a detailed investigation," said Assad Sarwar, an assistant political agent for Bajour.
At least one of those killed was a security officer and another was a hospital employee, Mohd Riaz, a health officer, said. Four people were reported injured.
The attack is the second in Khar involving a woman suicide bomber.
In December 2010, 45 people were killed when a woman wearing a woman blew herself up at a United Nations food distribution center, an action for which the Taliban took responsibility.