Only the man's 21-year-old sister survived the attack
A 35-year-old chartered accountant has murdered 14 members of his family, including seven children, with a butcher's knife.
Hasnin Anwar Warekar slit their throats then hung himself, police in India said.
Warekar's 21-year-old sister was the only survivor of the attack in Thane, 20 miles from the city of Mumbai, in the early hours of Sunday morning.
She was taken to hospital after neighbours heard her screaming for help.
Police broke open the front door and found the bodies.
Sky's Neville Lazarus said the dead included Warekar's wife, children, parents and siblings.
He said the family were "well-to-do" and news of the murders has shocked residents.
"We still haven't been able to speak with the attacker's 21-year-old sister, the lone survivor of the attack, who is in deep trauma at a city hospital," said Thane police spokesman Gajanan Laxman Kabdule.
Warekar attacked his family after apparently lacing their food with sedatives, say local reports.
"Prima facie evidence suggests that the accused bolted all the doors of the house and murdered his family while they were asleep, with a knife that we found near his body," Ashutosh Dumbre, joint commissioner of Thane police, told reporters.
He could not confirm whether the victims had been sedated, saying investigators were awaiting medical test results.
Mr Kabdule said the motive for the murders is "still sketchy".
According to the Press Trust of India news agency, the killings were sparked by a property dispute.