We administered the Seasonal Pattern Assessment Questionnaire (4) to 209 Caucasian individuals who were selected from the general population, which is described elsewhere (5). We computed global seasonality scores from the Seasonal Pattern Assessment Questionnaire, which are the sum of the scores on items pertaining to self-reported seasonal changes in mood and behavior and evaluated their serotonin transporter promoter repeat length polymorphism genotypes. Knowing that the short allele has been found to act in a dominant fashion (5), and on the basis of our results in patients with seasonal affective disorder (3), we hypothesized that those individuals who had at least one short allele would have higher global seasonality scores than those who were homozygous for the long allele. Our hypothesis was borne out: the global seasonality mean score for the long-long genotypes was 6.2, SD=3.9, and the mean score for the non-long-long genotypes was 8.9, SD=4.9 (t=3.65, df=207, p<0.001). There was no difference in seasonality mean scores between those who were heterozygous and those who were homozygous for the short allele (short-long=9.2, SD=4.6; short-short=7.9, SD=5.7). Thus, the serotonin transporter promoter repeat length polymorphism appears to influence the development of seasonality; higher seasonality scores occur in those with at least one short allele.