The mRNA levels of the nucleotide excision DNA repair gene ERCC1 and the base excision DNA repair gene OGG1 were quantified in 43 healthy volunteers in a dietary intervention trial as markers for the DNA repair capacity. Nine samples were collected from each subject over a period of 52 days. Sampling took place from January to May. The mRNA levels of OGG1 and ERCC1 correlated closely (r = 0.86, P << 0.0001) after normalization to either 18S ribosomal RNA or to beta-actin mRNA. The levels of OGG1 and ERCC1 mRNA were relatively constant within an individual with intra-individual correlation (R(2) = 0.45-0.46) in a General Linear Model. The amounts of ERCC1 and OGG1 relative to 18S RNA were doubled in May compared with January. This coincided with an increase in the monthly influx of sunlight from 18 MJ/m(2) in January to 242 MJ/m(2) in May. The mRNA levels of both ERCC1 and OGG1 were positively correlated to the average daily influx of sunlight in the previous 30 and 5 days (r = 0.49; r = 0.37, respectively, P << 0.001). There were no significant effects of the dietary interventions. The inter-individual variation was 5-10-fold, which is more than the observed 2-3-fold seasonal variation. Thus, despite seasonal variation of the individual mRNA levels, the inter-person variation is still far larger than the intra-person variation, supporting the use as biomarkers.