Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant disease of the fibrovascular tissue resulting in visceral vascular malformations and (muco-) cutaneous telangiectases with recurrent bleedings. The mechanism behind the disease is not fully understood; however, observations from HHT mouse models suggest that mechanical trauma may induce the formation of abnormal vessels. To assess the influence of environmental trauma (mechanical or light induced) on the number of telangiectases in patients with HHT, the number of telangiectases on the hands, face, and lips were counted on 103 HHT patients possessing at least three out of four Curaçao criteria. They were then surveyed for information concerning their dominant hand, exposure to sunlight, and types of regular manual work. Patients developed more telangiectases on their dominant hand and lower lip (Wilcoxon rank sum test: p < 0.001). Mechanical stress induced by manual work led to an increased number of telangiectases on patients’ hands (Mann–Whitney U test: p < 0.001). There was also a positive correlation between sun exposure and the number of telangiectases on the lips (Mann–Whitney U test: 0.027). This study shows that mechanical and UV-induced trauma strongly influence the formation of telangiectases in HHT patients. This result has potential implications in preventive measures and on therapeutic approaches for HHT.