Mental health hospitalization rates among U.S. children have been increasing locally and nationally in recent decades. Children in New York State (NYS) have also witnessed several collective traumatic events during the last two decades including the September 11, 2001, terrorist attacks (9/11), the Great Recession, and Hurricane Sandy (2012) and its aftermath. Decomposition of these rates into age, period, and cohort effects may help elucidate how large-scale collective traumatic events may be driving time trends. This study examined ageperiod-cohort effects in children and youth mental health hospitalizations in NYS from 1999-2013. Age effects followed a linear trend from age 5 years, B = −2.76, 95% CI [−3.48, −2.03)] up to age 15 years, B = 1.62, 95% CI [1.52, 1.73]. The largest period effects were noted in 2004, B = 0.36, 95% CI [0.28, 0.45], and in 2013, B = 0.31, 95% CI [0.15, 0.47], approximately 3 years after 9/11 and the Great Recession, respectively. The largest birth cohort effect was noted for children born in 1992-1995 (range: 0.29 for children born in 1992-0.27 for children born in 1995), suggesting that the birth cohorts who experienced the 9/11 attacks during middle childhood and the Great Recession during puberty are at increased risk of mental health hospitalizations compared to other birth cohorts. Approximately 13%-20% of children in the United States have a diagnosable mental health disorder (Merikangas et al., 2010; Perou et al., 2013). Psychiatric care accounts for 10% of all pediatric hospital admissions at general medical facilities and over $3 billion (USD) in hospital charges in the United States (Bardach et al., 2014). In New York City (NYC), psychiatric hospitalizations among adolescents have increased from 542 per 100,000 in 2000 to over 600 per 100,000 in 2013 (Mills & Davila, 2016). The reasons for the increase remain largely unknown but cannot be explained in terms of individual-level risk factors alone, suggesting short-and long-term impacts of contextual factors. During the 2000s, children in New York experienced several well-documented, large-scale traumatic events. The September 11 (9/11) attacks, a series of four coordinated terrorist attacks that included the destruction of the World Trade Center towers in NYC, occurred in 2001. The attacks killed 2,996 people that day (Arias, Anderson, Kung, Murphy, & Kochanek, 2003) and left an indelible impact on the American psyche (Centers for Disease Control and Prevention, 2002). The Great Recession, the deepest and longest economic decline in the postwar