Harris County Sheriff Ed Gonzalez said the two men walked into the medical clinic located at 15119 Wallisville Road around 2 a.m.
Deputies told ABC13 a vehicle was following the men when someone opened fire, shooting through the car's backseat.
One of the men was transported from the clinic to Ben Taub Hospital in critical condition. The second victim's condition is unknown.
Deputies were working to determine if the shooting was targeted or random.