In the past two decades, population-based health surveys have begun including measures of sexual orientation, permitting estimates of sexual orientation disparities in psychiatric morbidity and differences in treatment utilization. The present study takes advantage of the high-quality, comprehensive nationwide health registry data available in Sweden to examine whether psychiatric outpatient treatment for various diagnoses and antidepressant medication usage are greater in sexual minority individuals compared to their siblings. A longitudinal cohort study design was used with a representative random population-based sample in Stockholm, Sweden. Registry-based health record data on all specialized outpatient health care visits and prescription drug use was linked to a sample of 1154 sexual minority individuals from the Stockholm Public Health Cohort and their siblings. The main outcomes were treatment due to psychiatric diagnoses retrieved from nationwide registry-based health records. In analyses accounting for dependency between siblings, gay men/lesbians had a greater likelihood of being treated for mood disorder [adjusted odds ratio (AOR) 1.77; 99% confidence intervals (CI) 1.00, 3.16] and being prescribed antidepressants (AOR 1.51; 99% CI 1.10, 2.07) compared to their siblings. Further, bisexual individuals had a greater likelihood of any outpatient psychiatric treatment (AOR 1.69; 99% CI 1.17, 2.45) and being prescribed antidepressants (AOR 1.48; 99% CI 1.07, 2.05) as well as a greater likelihood of being treated for a mood disorder (AOR 1.98; 99% CI 1.33, 2.95) compared to their siblings. No difference in anxiety or substance use disorder treatment was found between any sexual minority subgroup and their siblings. The potential role of familial confounding in psychiatric disorder treatment was not supported for more than half of the outcomes that were examined. Results suggest that sexual minority individuals are significantly more likely to be treated for certain psychiatric disorders compared to their siblings. Future research is needed to understand mechanisms other than familial factors that might cause the substantial treatment differences based on sexual orientation reported here.