Background and Objectives: Partner notification and social network studies of infectious disease often involve interviewing people to elicit their sexual and/or drug injection partners. Incomplete reporting of partners in these contexts would significantly hamper efforts to understand and control the spread of sexually transmitted diseases, HIV, and other infections. There are many reasons why individuals might not name their partners in interviews. This study provides a comprehensive assessment of forgetting as a cause of incomplete reporting of sexual and injection partners. Study Design: One hundred fifty-six persons in Seattle, Washington, at presumed high risk for HIV recalled their sexual and/or injection partners in two interviews separated by 1 week or 3 months. Results: Repeated, nonspecific prompting elicited, on average, 10% of all partners recalled in an interview. Subjects displayed substantial forgetting of partners across partner types, recall periods, and four independent measurement approaches, with up to 72% of partners forgotten. The number of partners recalled and subjective assessment of forgetting are moderate to good predictors of the number of partners forgotten. Recalled and forgotten partners do not differ dramatically on any of several partner variables. Conclusions: Forgetting is a primary factor in the incomplete reporting of sexual and injection partners. Interviewers should prompt repeatedly to maximize recall of partners. Reinterviewing is currently the best method available for identifying partners as completely as possible and should be focused on individuals who report many partners and/or sense they have other partners they cannot recall.