There are two that I'm thinking of. One was AC797 on 6/2/83, a DC9 - a fire started in an aft lavatory quickly filled the cabin with smoke. By the time they got the plane down and evacuated, 23 out of 41 passengers died. This was about 20-30 minutes after they first thought something was wrong.

The other was a Varig flight 820 on a 707 on 7/11/73 - another aft lavatory fire, this time while the plane was already making its descent into Orly at 8000 feet when smoke was discovered. Six mintues later they made an emergency landing 5km away from the airport because they didn't think they could make it. 123 out of 124 passengers died, as did 7 out of 17 crew (most of the survivors were in the cockpit where they opened a window for ventilation).

Both of these were suspected to have been caused by someone smoking in the lav.