Summary Background Several pieces of evidence indicate that a complex relationship exists between constitutional telomere length (TL) and the risk of cutaneous melanoma. Although the general perception is that longer telomeres increase melanoma risk, some studies do not support this association. We hypothesize that discordant data are due to the characteristics of the studied populations. Objectives To evaluate the association of TL with familial and sporadic melanoma. Materials and methods TL was measured by multiplex quantitative polymerase chain reaction in leukocytes from 310 patients with melanoma according to familial/sporadic and single/multiple cancers and 216 age‐matched controls. Results Patients with sporadic melanoma were found to have shorter telomeres compared with those with familial melanoma. In addition, shorter telomeres, while tending to reduce the risk of familial melanoma regardless of single or multiple tumours, nearly trebled the risk of single sporadic melanoma. Conclusions This is the first time that TL has been correlated to opposite effects on melanoma risk according to the presence or absence of familial predisposition. Individual susceptibility to melanoma should be taken into account when assessing the role of TL as a risk factor. What's already known about this topic? The role of telomere biology in tumorigenesis is complex and influenced by multiple mechanisms, even acting in opposite directions. What does this study add? Constitutive telomere length is significantly different between familial and sporadic melanoma. Short telomeres increase the risk of single sporadic melanoma, but decrease that of familial melanoma. What is the translational message? Individual susceptibility to cancer should be taken into account when assessing the role of telomere length as a cancer risk factor. Linked Comment: Barrett. Br J Dermatol 2016; 175:865–866. Plain language summary available online