Although a large body of evidence suggests that diet may play an important role in cancer prevention, randomized controlled trials reported to date have not achieved sufficient increases in protective micronutrients and phytochemicals to adequately test the hypothesis that diet can reduce cancer risk. The Women’s Healthy Eating and Living (WHEL) Study, a randomized controlled trial of the role diet modification may play in future breast cancer events, introduced an innovative theory-based telephone counseling intervention to teach participants to consume a high fiber, low fat diet emphasizing vegetables and fruits rich in carotenoids and other potentially protective phytochemicals. This report examines the baseline to 12-mo changes in dietary intakes of 2970 participants, assessed through 24-h recalls and validated with plasma carotenoid concentrations. At 12 mo, the intervention group reported a significantly increased daily vegetable intake (+vegetable juice) of 7.1 servings (+82%) and fruit intake of 3.9 servings (+18%). Fiber intake increased from 3.04 to 4.16 g/(MJ. d), whereas energy from fat decreased significantly from 28.6 to 23.7%. Plasma carotenoid concentrations increased significantly, i.e., alpha-carotene (+223%); beta-carotene (+87%); lutein (+29%); and lycopene (+17%). In the comparison group, dietary intake and plasma carotenoid concentrations were essentially identical to those of the intervention group at baseline and were unchanged at 12 mo. The WHEL Study showed that a telephone counseling intervention can achieve major increases in micronutrient- and phytochemical-rich vegetables, fruit and fiber intakes, enabling an investigation of the potential cancer preventive effects of these food components.