The victims were hospitalized after eating Amanita phalloides, better known as "death cap" mushrooms, according to hospital spokesman Kevin McCormack.
McCormack said the families of the victims declined to release further information about the incident or the victims' conditions except that they remained in the hospital's intensive care unit as of 3:45 p.m.
The "death cap" mushroom is found in wooded and grassy areas throughout Northern California, and is particularly prevalent after heavy rains.
The mortality rate at California Pacific Medical Center among patients who have eaten the mushrooms is about 10 percent, according to hospital officials.
Visitors and immigrants from other countries seem to be at particular risk of ingesting the mushrooms because they look like edible mushrooms from their homeland, so much so that experienced mushroom hunters have made the mistake of eating them, according to officials.
Dr. Carrie Frenette, the physician who is treating the patients, advised in a statement, "Never use wild mushrooms in food unless you are absolutely certain that they are safe. If there is any suspicion that poisonous mushrooms have been eaten, do not wait for symptoms to begin, instead call the poison center immediately."
The California Poison Control System can be contacted at (800) 876-4766 or by going to www.calpoison.org.