ABSTRACT: BACKGROUND:Older lesbian, gay, and bisexual (LGB) adults are an underserved and understudied population that experience specific health disparities. The intersection of aging and chronic medical disease with a higher risk for substance use and mental illness may place older LGB adults at risk for co-occurring conditions and resulting comorbidity. Understanding multimorbidity among older LGB adults may help inform interventions to reduce disparities in health outcomes. METHODS:Data come from the 2015 to 2017 National Surveys on Drug Use and Health (n?=?25,880). We first determined whether sexual orientation was associated with reporting: past-year drug use, mental illness, and/or 2 or more chronic medical diseases. We then determined whether sexual orientation was associated with reporting co-occurrence of these conditions. This was done using multivariable logistic regression. Analyses were stratified by gender. RESULTS:Compared to heterosexual men, gay men were at increased odds for reporting 2 or more chronic medical diseases (adjusted odds ratio [aOR]?=?2.18, 95% confidence interval [CI]?=?1.48, 3.21), and gay (aOR?=?1.79, 95% CI?=?1.09, 2.93) and bisexual men (aOR?=?3.53, 95% CI?=?2.03, 6.14) were at increased odds for reporting mental illness. Gay men (aOR?=?2.95, 95CI?=?1.60, 5.49) and bisexual men (aOR?=?2.84, 95% CI?=?1.58, 5.08) were at increased odds of reporting co-occurring conditions. Compared to heterosexual women, bisexual women were at increased odds for past-year drug use (aOR?=?4.20, 95% CI?=?2.55, 6.93), reporting mental illness (aOR?=?1.94, 95% CI?=?1.03, 3.67), and reporting co-occurring conditions (aOR?=?3.25, 95%?=?1.60, 6.62). CONCLUSIONS:Middle-aged and older LGB adults in the United States are at high risk for experiencing co-occurring drug use, mental illness, and/or medical multimorbidity. Interventions for older sexual minority populations are needed to reduce disparities.