Objective: New findings, that relate poor foetal growth to long-term outcomes, highlight the need to understand more about the nature of women's diets before and during pregnancy. This study examines the influence of sociodemographic and anthropometric factors on the quality of the diets of young women in the UK. Design: Diet was assessed by an interviewer-administered food frequency questionnaire. A single diet score was calculated for each woman using the first component defined by principal components analysis. Setting: Southampton, UK. Subjects: A total of 6125 non pregnant women aged 20-34 y. Results: The diets of women with low diet scores were characterised by low intakes of fruit and vegetables, wholemeal bread, rice and pasta, yogurt, and breakfast cereals, but high intakes of chips and roast potatoes, sugar, white bread, red, and processed meat and full-fat dairy products. Educational attainment was the most important factor related to the diet score. In all, 55% (95% CI 50-59%) of women with no educational qualifications had scores in the lowest quarter of the distribution, compared with only 3% (95% CI 2-4%) of those who had a degree. Smoking, watching television, lack of strenuous exercise, and living with children were also associated with lower diet scores. After taking these factors into account, no other factor including social class, the deprivation score of the neighbourhood, or receipt of benefits added more than 1% to the variance in the diet score. Conclusions: Poor achievement at school defines a substantial group of women in the UK who may be vulnerable. Many of these women have poor diets that are not simply a result of the level of deprivation in their neighbourhood, or of living at a level of poverty that entitles them to benefits. We suggest that it is a priority to identify and to address the barriers that prevent these women from improving the quality of their diets.