Smoking is a direct cause of fatal brain haemorrhages, a pioneering study of more than 13,000 sets of twins has shown.
Researchers in Finland claim to have "confirmed" that smoking leads to the catastrophic brain events and compare their discovery to groundbreaking research which showed smoking caused lung cancer.
Ilari Rautalin, the principal investigator, says that the finding is "historic" and "enormously important."
The doctoral student, who has a medical degree, said: "Our findings confirm, for the first time, previous suspicions of an actual causal relation between smoking and fatal brain haemorrhage."
The team, from the university of Helsinki, in Finland, say their results show it is specifically smoking that caused brain haemorrhages in one twin when the other non-smoking twin did not.
They add that their findings make it likely that smoking causes a significant share of brain haemorrhages in both men and women.
Dr Jaakko Kaprio, professor of genetic epidemiology, explained why the team used twins.
He said: "It's practically impossible to carry out a conventional clinical trial proving causality with regard to smoking, as it would require exposing healthy individuals to smoking and, subsequently, to many adverse health effects associated with smoking.
"This is why twin studies are an effective way of demonstrating direct causalities related to smoking."
