Smoking just one to four cigarettes a day almost triples a person's risk of dying of heart disease, according to Norwegian researchers.
Doctors warn that any amount of smoking is dangerous
Their work suggests the health impact is stronger for women and that even "light" smokers face similar diseases to heavier smokers, including cancer.
The team tracked the health and death rates of almost 43,000 men and women from the mid 1970s up to 2002.
Their findings appear in the journal Tobacco Control.
Compared with those who had never smoked, the men and women who smoked between one and four cigarettes a day were almost three times as likely to die of coronary artery disease.
Among women, smoking one to four cigarettes daily increased the chance of dying from lung cancer almost five times.
Men who smoked this amount were almost three times as likely to be killed by lung cancer.
However, due to the relatively small number of men that this applied to in the study sample, this finding could have been due to chance.
So-called "light" smokers were also found to have a significantly higher risk of dying from any cause - 1.5 times higher generally - than those who had never smoked, when researchers looked at deaths among those studied over the duration of the research.
Death rates from all causes rose as the number of cigarettes smoked every day increased.
The researchers believe their conclusions are accurate, even though they had to estimate the projected impact of smoking one to four cigarettes for five years in those light smokers who had smoked for less time.
This indicated that the risk of death from coronary artery disease for both sexes would have been 7% higher, and the risk of lung cancer would have been 47% higher in women.
A significant proportion of the light smokers had also increased their daily consumption over the period of the study. However, this had not exceeded nine cigarettes a day.
Author Dr Kjell Bjartveit also pointed out that it was not possible to tell from the findings what impact sporadic smoking - such as a few cigarettes on a Saturday night out - might have on health.
Dr Ken Denson of the Thame Thrombosis and Haemostasis Research Foundation questioned the validity of the figures.
He said other large studies had not found that smoking fewer than 10 cigarettes daily increased the risk of heart disease.
'No safe level'
Amanda Sandford from Action on Smoking and Health said the conclusions were clear.
"This study should dispel the myth once and for all that smoking just a few cigarettes a day won't do you any harm.
"Quite simply, there is no safe level of smoking."
A spokesman from the British Medical Association said: "All smokers are putting their health on the line when they smoke - even if they only define themselves as social smokers.
"The only way to protect smokers from heart disease, cancer and other killer diseases is to quit completely."
The Department of Health estimates 106,000 people die every year in the UK as a direct result of smoking. It said quitting was the only way to avoid the serious health risks.
Jean King of Cancer Research UK said: "Although more research is needed, this study suggests that the health implications for 'light smokers' are much more serious than previously thought.
"This is particularly worrying as a third of smokers in the UK - an estimated 3.7 million people - smoke less than 10 cigarettes a day."