Delivery of culturally competent care toward lesbian, gay, bisexual, and transgender (LGBT) patients depends on how health-care providers (HCPs) communicate with them; however, research about knowledge, attitude, and behavior of HCPs toward LGBT patients is scant. The objectives of our study were to describe oncology HCPs' knowledge and examine if beliefs about LGB and transgender patients mediate the effects of LGBT health-care knowledge on open communication behaviors with LGB and transgender patients, respectively. A total of 1253 HCPs (187 physicians, 153 advance practice professionals (APPs), 828 nurses, and 41 others) at a Comprehensive Cancer Center completed an online survey that included the following measures:LGBT health-care knowledge, beliefs, communication behaviors, willingness to treat LGBT patients, encouragingLGBT disclosure, and perceived importance of LGBT sensitivity training. Only 50 participants (5%) correctly answered all 7 knowledge items, and about half the respondents answered 3 (out of 7) items correctly. Favorable beliefs about LGBT health care mediated the effect of higher LGBT health-care knowledge on open communication behaviors with transgender patients, controlling for effects of type of profession, religious orientation, gender identity, sexual orientation, and having LGBT friends/family. The results of this study demonstrated an overall lack of medical knowledge and the need for more education about LGBT health care among oncology HCPs.Lesbian, gay, bisexual, transgender (LGBT) is an umbrella term that refers to sexual minority (e.g., lesbian, gay, bisexual) and gender minority (e.g., transgender, nonbinary, genderqueer) populations. While estimates vary, population surveys suggest between 5.2 and 9.5 million adults in the United States identify as LGBT (Gates, 2014). Barriers to equitable health care are multifactorial and include the experiences of these populations with the health-care system (Institute of Medicine (IOM), 2011). Evidence suggests that LGBT populations are at greater risk for breast, prostate, anal, cervical, colorectal, endometrial, and lung cancers when compared to the general population (Quinn et al., 2015), due to higher