Intraduction: There is known to be a relationship between vitamin D level and more aggresive breast cancer subtypes, especially triple-negative breast cancer (TNBC). It was reported that sunlight exposure has an effect on the prognosis of patients with cancer, possibly related to the conversion of vitamin D to its active form with sunlight. We aimed to evaluate the effect of sunlight exposure on patients with TNBC. Materials-Methods: A total of 1,167 patients with breast cancer from two different regions of Turkey (Antalya and Kayseri, regions having different climate and sunlight exposure intensity over the year) were analysed retrospectively. The ratio of patients with TNBC was identified in those two regions. Results: The ratio of patients with TNBC was 8% and 12% for Kayseri and Antalya regions, respectively, and this difference between the two groups was statistically significant (p=0.021). Discussion: Sunlight exposure may be associated with more prevalent TNBC. This finding should be investigated with a prospective study.