At least four people died this month after eating "sugihiratake" mushrooms, health officials said Friday.
Three of the victims died in Niigata Prefecture, where they were among 11 people in their 50s to 80s who fell ill, officials said.
All 11 people, who already had poor kidneys, came down with acute fever and convulsions about two weeks after eating the wild white mushrooms, Niigata health official Kazuo Ishigami said.
"It is a type of mushroom that people usually eat around here and is said not to be poisonous," he said. "But the link (to the deterioration of their health) is that they all ate the mushrooms.
"Another point is that all of the 11 had poor kidney function or were on dialysis."
Another two people in their 60s and 70s died of a similar fever this month in neighboring Yamagata Prefecture, a local official said.
Both were on dialysis and one of them is known to have eaten the same variety of mushroom, he said.
"There is also the theory that something else may have been stuck to the mushroom. It might also have been an infection of some kind," he said.
Ishigami said local officials were in contact with the health ministry.