Healthy adult volunteers (no. = 208), men and women aged 30 to 65 years, participated in a 12-week study on dietary fat modification plus oat product ingestion (60 gm/day) to test whether moderate daily intake of oat bran and oatmeal enhanced serum lipid response. During weeks 0 to 6, all participants followed the American Heart Association fat-modified eating style. At 6 weeks, participants were randomly assigned to one of three groups. All participants continued to follow the fat-modified eating pattern; groups 1 and 2 were asked during weeks 7 to 12 to consume two servings of either oat bran or oatmeal per day, for a total of 60 gm/day isocalorically substituted for other carbohydrates. Group 3 ingested no oat products. At baseline, the group mean cholesterol level was 208.4 mg/dl. After 6 weeks of dietary fat intervention, the level was 197.6--a fall of 10.8 mg/dl (5.2%). At 12 weeks, the mean serum cholesterol level fell further, by 5.6, 6.5, and 1.2 mg/dl for groups 1, 2, and 3, respectively. Group mean weight loss was small--1.9 lb during the first 6 weeks and 0.6 to 0.8 for the three groups during weeks 7 to 12. Reported oat product ingestion was 39 and 35 gm per person per day, respectively, for groups 1 and 2 (2.2 and 1.4 servings per person per day, respectively). Dietary fat composition remained similar among the three groups during weeks 7 to 12. Pooled results indicated that the addition of oat products at a moderate and practical level enhanced serum lipid response (p less than .05) to a fat-modified eating pattern among free-living adults.