Objective: To describe basic nonsuicidal self-injury (NSSI) characteristics and to explore sex differences. Methods: A random sample from 8 universities were invited to participate in a Web-based survey in 2006-2007; 38.9% (n = 14,372) participated. Analysis assessed sex differences in NSSI prevalence, practices, severity, perceived dependency, and help-seeking; adjusted odds ratios for NSSI characteristics were calculated by sex status. Results: Lifetime NSSI prevalence rates averaged 15.3%. Females were more likely than males to self-injure because they were upset (adjusted odds ratio [AOR] = 1.6; 95% confidence interval [CI] = 1.3-2.1) or in hopes that someone would notice them (AOR = 1.6, 95% CI = 1.1-2.7). Males were 1.6 times (95% CI = 1.2-2.2) more likely to report anger and 4.0 times (95% CI = 2.3-6.8) more likely to report intoxication as an initiating factor. Sexual orientation predicted NSSI, particularly for women (Wald F = 8.81, p [less than or equal to] 0.000). Only 8.9% of the NSSI sample reported disclosing NSSI to a mental health professional. Conclusions: NSSI is common in college populations but varies significantly by sex and sexual orientation. NSSI disclosure is low among both sexes. (Contains 3 tables.)