Icelandic privacy prevents revealing the cause of death, but the chessbase site says they knew he had kidney disease. Also on the Cb website is an interview with Miyoko Watai, his wife. She said he prefered the Oriental approach--using herbs and such--to western medicine.
He did it his way. Not surprising.
One of my favorite quotes is from Svetozar Gligoric's book about the 1972 Fischer-Spassky match: "After all, its how well one plays chess thats important."