Paramedics reportedly took the 82-year-old actor to UCLA Medical Centre after severe chest pains prompted a 911 call.
Leonard Nimoy, who found fame in the role of Star Trek's Spock, announced on Twitter back in January that he had a serious lung disease, which had been caused by him smoking thirty years ago.
“I quit smoking 30 yrs ago,” he tweeted. “Not soon enough. I have COPD. Grandpa says, quit now!! LLAP.”
LLAP, as many Star Trek fans will no doubt be aware, stands for Spock's catchphrase: Live long and prosper.
Chronic obstructive pulmonary disease, according the National Heart, Lung and Blood Institute, blocks airflow, produces excess mucus and makes it hard for sufferers to breathe.
It has not yet been revealed how Leonard, who officially retired from acting in 2010, is doing in hospital.
A representative for the actor said: "We don't have any comment at this time."