BACKGROUND: Food processing alters food from its natural state for safety, convenience, taste or palatability. Previous research suggests that industrially processed foods, and diets high in these products, tend to be less healthful. However, most previous work is based on household, rather than individual-level, data. Little has been reported on the relationship between processed food consumption and markers of health; or on socio-demographic correlates of processed food consumption. Our objective was to describe: the nutritional content of foods classified according to degree of processing; the nutritional content of diets with different relative intakes of processed foods; the socio-demographic characteristics of individuals with different relative intakes of processed foods; and the association between intake of processed foods and body weight. METHODS: Secondary analysis of data from the UK National Diet and Nutrition Survey (2008-12), a large national cross-sectional study of diet. Dietary information was collected using four-day, unweighed, food-diaries. Foods were classified as: unprocessed or minimally processed (MPF; foods with no processing or mostly physical processes applied to single whole foods), processed ingredients (PI; extracted and purified components of single whole foods), or ultra-processed food products (UPF; products produced from industrial combining of MPF and PI). RESULTS: Two thousand one hundred seventy four adults were included. MPF and diets high in these foods, had the most healthful nutritional profile. UPF did not necessarily have the least healthful nutritional profile, but diets high in these foods did. Women, and older adults consumed more energy from MPF, and less from UPF. Those living in lower occupation social class households consumed less energy from MPF, but no more from UPF. Only higher intake of PI was consistently, inversely, associated with body weight. CONCLUSIONS: This is the first study to explore correlates of processed food consumption, using individual-level data from a large, national sample. Although higher intakes of MPF and lower intakes of UPF were associated with the most healthful dietary profiles, only intake of PI was consistently associated with body weight. Consumption of UPF varied by age and gender, but, unexpectedly, not by occupational social class. Longitudinal work is required to confirm relationships with health markers.