We investigated the relationship between geography and incidence of coronary heart disease, looking at deficiency of sunlight and thus of vitamin D as a factor that might influence susceptibility and thus disease incidence. Sunlight deficiency could increase blood cholesterol by allowing squalene metabolism to progress to cholesterol synthesis rather than to vitamin D synthesis as would occur with greater amounts of sunlight exposure, and the increased concentration of blood cholesterol during the winter months, confirmed in this study, may well be due to reduced sunlight exposure. We show evidence that outdoor activity (gardening) is associated with a lower concentration of blood cholesterol in the summer but not in the winter. We suggest that the geographical variation of coronary heart disease is not specific, but is seen in other diseases and sunlight influences susceptibility to a number of chronic diseases, of which coronary heart disease is one.