Multiple sclerosis and lung cancer: an unexpected inverse association.

Multiple sclerosis is associated with a decreased risk of cancer. Smoking is a risk factor both for multiple sclerosis and lung cancer. We performed a meta-analysis on studies of cancer frequency in multiple sclerosis. Surprisingly, we found that the risk of lung cancer is reduced in multiple sclerosis [odds ratio 0.67 (95% confidence interval 0.59-0.76) P < 0.00001]. Since this does not appear to be secondary to altered smoking behaviour, we hypothesise that this may be secondary to altered immune surveillance.