Several studies have identified potential detrimental sequelae of cholesterol and fat-lowering interventions in randomized trial. Little research has been published to document changes in mental health in women as a result of fat and cholesterol lowering interventions to prevent chronic disease. This paper examines the relationships among changes in dietary fat consumption and mental health in the Women's Health Trial, a randomized, controlled trial to determine whether lowering fat consumption to 20% of daily calories could reduce the incidence of breast cancer in women ages 45-69 years. Assessments were made at baseline and at the 12-month follow-up of several aspects of quality of life, including negative and positive affect and past, present, and future perceptions of health. Mental health variables were measured by the Mental Health Inventory, a standardized scale used in the Medical Outcomes study. Dietary intake was assessed for all subjects with the use of semiquantitative food frequency questionnaires. The change in mental health values (follow-up minus baseline) was significantly different between intervention and control groups for three of the four psychological variables: (a) anxiety; (b) depression; and (c) vigor. In all three cases, the direction of the change for intervention women was positive. Neither randomization assignment nor percent of calories from fat at the follow-up visit were significant predictors of mental health at the 1-year follow-up. Cholesterol changes were not related to levels of mental health variables in a sample of the women. These data indicate that lowering fat in the diets of healthy women does not produce overall lowering of any mental health variables.(ABSTRACT TRUNCATED AT 250 WORDS)