Background: Approximately 70% of US adults currently receiving opioid agonist treatment (OAT) for opioid use disorder are of reproductive age. Among women receiving OAT at risk of having an unintended pregnancy, typically less than half report any current contraceptive use compared to 90% in the general population. In addition, the rate of unintended pregnancy among women receiving OAT is disproportionately higher than the general population (~80% vs. 45%, respectively). Lack of knowledge about family planning may be contributing to decreased rates of contraceptive use and increased rates of unintended pregnancy among women receiving OAT. Method: Participants were a convenience sample of women and men receiving OAT or a comparison group receiving primary care (PC) services. Family planning knowledge was assessed with the recently validated Contraceptive Knowledge Assessment (CKA), a self-administered 25-question multiple-choice survey. A two-way ANOVA, with fixed factors (i.e. patient sample and sex), compared the total number of correct responses for all questions and five more specific content areas (p<.05). Results: Overall, 332 participants completed this survey. The mean percent of total correct responses was significantly lower in the OAT sample (n=167) compared to the PC sample (n=165), 47% vs. 53% correct, respectively (p<.001) or approximately 1.5 questions less. The mean percent of correct responses in four of the five content areas was also lower among the OAT sample compared to the PC sample (ps<.01). The mean percent of total correct responses was significantly higher among women (n=169) than men (n=163), 56% vs. 44% correct, respectively (p<.0001) or approximately 3 questions more. The percent of correct responses in four of the five content areas was also higher among women than men (ps<.01). Conclusion: Given the substantial discrepancy in rates of contraceptive use and unintended pregnancy between individuals receiving OAT and the general population, it is somewhat unexpected that individuals receiving OAT did not have lower levels of family planning knowledge, although patients in both samples only answered approximately 50% of the questions correctly. Results from the present study suggest deficits in family planning knowledge, while statistically significant, may be less clinically so. Overall, lack of family planning knowledge is likely only playing a small role in population differences in contraceptive use and unintended pregnancy and interventions aimed at decreasing these differences will need to address other barriers to accessing family planning services and utilizing contraception in this population.