Telomere length is important in tumorigenesis. Using quantitative real-time PCR, we prospectively measured relative telomere length in a nested case-control study within the Nurses' Health Study: 218 melanoma cases, 285 squamous-cell carcinoma (SCC) cases, 300 basal-cell carcinoma (BCC) cases, and 870 controls. We observed that shorter telomeres were associated with a decreased number of moles (P=0.002) and a decreased risk of melanoma. Women in the second and first quartiles, those with the shortest telomere length, had an odds ratio (OR) for melanoma of 0.54 (95% confidence interval (CI), 0.29-1.01) and 0.59 (95% CI, 0.31-1.13), respectively, compared with those in the fourth quartile (P, trend=0.09). There was no clear trend between telomere length and SCC risk. In contrast, we found that shorter telomere length was associated with an increased risk of BCC. Compared with those in the fourth quartile, women in the first quartile had an OR of 1.85 (95% CI, 0.94-3.62) (P, trend=0.09). The opposing associations observed should be interpreted with caution, and further research is needed to confirm these possible associations.