We aim to examine temporal trends of orthopedic operations and opioid-related hospital stays among seniors in the nation and states of Oregon and Washington where marijuana legalization was accepted earlier than any others.As aging society advances in the United States (U.S.), orthopedic operations and opioid-related hospital stays among seniors increase in the nation.A serial cross-sectional cohort study using the healthcare cost and utilization project fast stats from 2006 through 2015 measured annual rate per 100,000 populations of orthopedic operations by age groups (45-64 vs 65 and older) as well as annual rate per 100,000 populations of opioid-related hospital stays among 65 and older in the nation, Oregon and Washington states from 2008 through 2017. Orthopedic operations (knee arthroplasty, total or partial hip replacement, spinal fusion or laminectomy) and opioid-related hospital stays were measured. The compound annual growth rate (CAGR) was used to quantify temporal trends of orthopedic operations by age groups as well as opioid-related hospital stays and was tested by Rao-Scott correction of χ for categorical variables.The CAGR (4.06%) of orthopedic operations among age 65 and older increased (P < .001) unlike the unchanged rate among age 45 to 64. The CAGRs of opioid-related hospital stays among age 65 and older were upward trends among seniors in general (6.79%) and in Oregon (10.32%) and Washington (15.48%) in particular (all P < .001).Orthopedic operations and opioid-related hospital stays among seniors increased over time in the U.S. Marijuana legalization might have played a role of gateway drug to opioid among seniors.