It has been stated recently that atmospheric acid haze containing sulfur dioxide and sulfate crystals may lead to vitamin D deficiencies in exposed populations and increase breast cancer mortality, especially in Montréal. In view of the implications of this hypothesis, we have done a case-control study within the same cohort of women attending the Canadian National Breast Screening Study to check whether a depletion of vitamin D could be detected in the diet of breast cancer patients (n = 108) as compared to controls (n = 322) or a random sample of 1,141 women in our Montréal centre as well as 40 breast cancer cases for which precise food intake data were available by way of a 24-hour dietary journal. The mean daily intake of vitamin D of breast cancer cases was 1.65 +/- 2.48 I.U./kg while in 322 controls matched for age the mean intake was 1.34 +/- 1.17 (S.D.). It has to be pointed out that in the 5 years before diagnosis, cancer patients had not increased significantly their consumption of foods rich in vitamin D, namely milk, cheese, yoghurt, butter and fish. It has also to be stressed that, in relation to the recommended daily intake, twice as many breast cancer patients than controls had a higher consumption of vitamin D.