Sullivan's research focuses on HIV among men who have sex with men, including behavioral research, interventions, and surveillance. Previously, Sullivan worked as the chief of the Behavioral and Clinical Surveillance Branch in the HIV Division of the Centers for Disease Control and Prevention, implementing HIV research studies and surveillance systems. He is the principle investigator of NIH-funded studies to determine reasons for black/white disparities in HIV among MSM and to develop and test a couples-HIV-testing intervention for male couples.