A teenager was taken to hospital after being shot in the chest in Greenford.
The 14-year-old boy is believed to have been wounded in Sudbury Heights Avenue early on Tuesday (30) afternoon.
Police were first alerted just after 2pm by a member of staff at the hospital after the victim was brought in by two unknown men.
The teenager was then transferred to another hospital in central London where he remains in a stable condition.
Officers from Operation Trident - which focuses on gun crime in the black community - are investigating but no arrests have been made.
A spokesman for the Metropolitan police said: "We are appealing for anyone who was in the Sudbury Heights Avenue area of Greenford between 1pm and 2pm to come forward.
"We are also very keen to speak to the two men who took the youth to hospital as they may have vital information to assist the investigation."
Anyone with information about the incident is asked call Operation Trident on 0208 733 4774 or Crimestoppers anonymously