Researchers found that smoking and HPV infection together increase the risk of cervical cancer more than would be expected based on the amount of risk associated with each risk factor alone.
The study included data on HPV infection status collected during cervical smears from 738 women, 422 of whom went on to develop cervical cancer an average of nine years later.
The women also completed interviews, giving data on other risk factors including smoking.
Women who tested positive for HPV-16 infection were 8.4 times as likely to develop cervical cancer as those testing negative. Current smokers were 1.9 times as likely to develop the cancer as non-smokers.
But women who were smokers and tested positive for HPV infection were more than 14 times as likely to develop cervical cancer as non-smokers with no HPV infection.
They also found that women with a high viral load of HPV were more likely to develop cancer if they smoked.
In non-smokers, a high viral load was associated with a 5.9-fold increased risk, but in smokers it increased the risk 27-fold.
Lead researcher Dr Anthony Gunnell, from the department of medical epidemiology and Biostatistics at the Karolinska Institute, Stockholm, said: ‘We were surprised to see this dramatically increased risk among women with high viral loads who smoked.
‘Our study implies a synergistic action between HPV and smoking. This would put them in a risk group worthy of careful monitoring.'