"Krisis of the Krimson Kryptonite, Part 1 of 4"
Continued in The Adventures of Superman #472.
While mourning the death of his son, Lex is visited by Mxyzptlk and receives a piece of red Kryptonite. Confronting Superman with the Kyrptonite robs the Man of Steel of his
powers and a fist fight ensues.
Lex's mystery ailment is finally revealed as cancer caused by Kryptonite exposure.
Lois reveals that her mother is dying.