A 17-year-old boy was dragged off his bike and had his head stamped on by two men, leaving him with a fractured skull.
He had been cycling with a friend in Portsmouth on Thursday night when the men approached from beside a silver van, police said.
His friend was hit with a "long object" but managed to ride away and raise the alarm. The attackers then fled the scene on Laburnam Grove.
Police have appealed for witnesses.
Hampshire Constabulary said the attack, which also left the boy with a broken nose and possible broken jaw, happened at about 22:20 BST.
The man who stamped on the victim's head was described as a slim, white man in his late 20s or early 30s who was about 5ft 8ins and had short brown spiked hair.
The other man was described as white, about 6ft 1ins tall with a stocky build.