The authors examined the feasibility of using plasma carotenoids and ascorbic acid as markers of compliance for dietary intervention trials aimed at increasing the quantity and variety of the fruit and vegetable intake of free-living individuals. Nineteen former cancer patients who had been successfully treated for a stage I or II squamous cell carcinoma of the mouth, pharynx, larynx, or lung were recruited. Subjects served as their own controls. However, in order to detect any seasonal trends, 4 individuals among the 19 were randomized to a nonintervention group. Subjects in the intervention group were counseled by dietitians with the goal of increasing their intake of fruits and vegetables to eight servings/day (1 serving each of dark green vegetables, yellow-orange vegetables, tomato products, and other vegetables; 3 servings of vitamin C-rich fruits; and 1 serving of other fruits). Subjects in the nonintervention group were advised to follow their usual diet. Three-day measured food records kept at base line and after 3 months of intervention, as well as unannounced 24-h dietary recalls, documented an increase in mean fruit and vegetable intake from 4.2 to 9.5 servings daily in the intervention group. A concomitant increase of 29% was observed in total plasma carotenoids (P = 0.02), with increases of 25% for plasma lycopene (P = 0.06), 31% for plasma lutein (P = 0.002), 39% for plasma beta-carotene (P = 0.01), and 57% for plasma alpha-carotene (P = 0.01). Mean plasma levels of ascorbic acid increased by 27% (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)