Houston Police Department officers said a witness told them the motorcyclist appeared to have been involved in an altercation with an SUV driver on the Sam Houston Parkway near the North Freeway.
Once the vehicles reached the top of the flyover, HPD Lt. R. Willkens said the motorcyclist crashed and fell into a grassy area.
"Fell quite a long way down," Willkens said. "Very, very hard fall."
At last check, police said the motorcyclist is in critical condition and had to undergo surgery.
The driver of the SUV fled the scene.
Willkens said they're working to find out who the SUV driver was. The injured victim has not been identified.
Anyone with information can contact the Vehicular Crimes Division with HPD at 713-247-4072.