Plasma samples were collected at monthly intervals for a period of 1 year from a group of healthy nonsmoking men and women (n = 21) living in Honolulu, HI. Analysis of plasma cholesterol and triglyceride levels showed marked seasonal variations, with higher mean levels in winter months and lower values in the summer. Cholesterol and triglycerides were highly and inversely correlated with plasma levels of the provitamin A carotenoids. Mean beta- and alpha-carotene levels were highest in late summer and fall. Plasma retinol levels were significantly lower in the summer and higher in the winter. Variations (either between individuals or seasonally) in plasma retinol were unrelated to plasma provitamin A carotenoid levels. Plasma levels of alpha-tocopherol, gamma-tocopherol, beta-cryptoxanthin, and lutein were also higher in the winter and lower in the summer. Significant seasonal correlations, both positive and negative, with environmental variables, such as temperature, solar UV radiation, and rainfall, are noted for many of these plasma micronutrients. The number of samples required to accurately characterize long-term plasma levels for an individual generally ranged from 1 to 4. However, plasma retinol levels exhibited the highest ratio of intra- to interindividual variability, suggesting the need for multiple sampling (> 8 samples) for this micronutrient. Some of this variability for retinol was associated with seasonal changes. Assessment by a diet history of food and supplement intake of micronutrients and phytochemicals for 1 year showed good agreement with 1-year mean plasma levels for most carotenoids, vitamin C, and alpha-tocopherol. Retinol, gamma-tocopherol, cholesterol, and triglyceride levels in plasma were unrelated to estimates of dietary intake.(ABSTRACT TRUNCATED AT 250 WORDS)