IMPORTANCE: In the United States and Minnesota, melanoma incidence is rising more steeply among women than men younger than 50 years. To our knowledge, no study has examined age- and sex-specific associations between indoor tanning and melanoma to determine if these trends could bedue to greater indoor tanning use among younger women. OBJECTIVE: To examine associations between indoor tanning and melanoma among men and women younger than 50 years. DESIGN, SETTING, AND PARTICIPANTS: Population-based case-control study conducted in Minnesota of 681 patients (465 [68.3%] women) diagnosed ashaving melanoma between 2004 and 2007, and 654 controls (446 [68.2%] women), ages 25 to 49 years. EXPOSURE: Indoor tanning, defined as any use, first age of use, and total sessions. MAIN OUTCOMESAND MEASURES: Crude and adjusted odds ratios (ORs) and 95% CIs were calculated for melanomain relation to indoor tanning exposure for men and women by diagnosis or reference age (<30, 30-39, 40-49 years). Sex-specific associations for indoor tanning and melanoma by anatomic site were examined. RESULTS: Compared with women aged 40 to 49 years, women younger than 40 years initiated indoor tanning at a younger age (16 vs 25 years, P <.001) and reported more frequent indoor tanning (median number of sessions, 100 vs 40, P <.001). Women younger than 30 years were 6 times more likely to be in the case than the control group if they tanned indoors (crude OR, 6.0; 95% CI, 1.3-28.5). Odds ratios were also significantly elevated among women, ages 30 to 49 years (adjusted OR, 3.5; 95% CI, 1.2-9.7 for women 30-39 years; adjusted OR, 2.3; 95% CI, 1.4-3.6 for women 40-49 years);a dose response was observed among women regardless of age. Among men, results byage were inconsistent. The strongest OR for indoor tanning by anatomic site was for melanomas arising on the trunk of women (adjusted OR, 3.7; 95% CI, 1.9-7.2). CONCLUSIONS AND RELEVANCE: Indoor tanning is a likely factor for the steeper increase in melanoma rates inthe United States among younger women compared with men, given the timing of when women initiated indoor tanning relative to diagnosis. The melanoma epidemic can be expected to continue unless indoor tanning is restricted and reduced.