Abstrak: The incidences of malignant melanoma in situ (MMIS) and invasive malignant melanoma are rising in the United States, but few studies have examined risk factors for MMIS. We evaluated the risk of MMIS according to the host phenotype and the ultraviolet index of the state of residence. Prospective data were collected via biennial questionnaires from 250,151 women and men aged ≥20 years in the Nurses' Health Study (1980-2008), the Nurses' Health Study 2 (1989-2009), and the Health Professionals Follow-up Study (1986-2008). During 7,144,820 person-years of follow-up, 888 incident MMIS lesions occurred, representing 33% of all incident malignant melanoma. Meta-analysis across the cohorts demonstrated that the presence of multiple nevi on the extremities conferred the highest relative risk for MMIS (relative risk = 3.18, 95% confidence interval: 2.59, 3.90). Family history of melanoma, number of severe sunburns, sunburn susceptibility, hair color, and Fitzpatrick skin types I, II, and III were significantly associated with an increased risk of MMIS. Conversely, the ultraviolet index of the state of residence at birth, at age 15 years, and at age 30 years was not associated with increased risk of MMIS. Continued study of MMIS and associated risk factors will help identify persons who are most at risk and elucidate the role of MMIS within the spectrum of cutaneous melanoma.