Concurrent sexual partnerships contribute to STI/HIV transmission. STI/HIV risk may differ depending upon the circumstances and motivations surrounding concurrency. Greater understanding of motivations for concurrency initiation may assist prevention programs. However, most concurrency studies are cross-sectional, which limits ability to be certain which factors preceded concurrency initiation. Also, concurrency is a function of relative timing of when partnerships begin and end. But do people accurately report the dates on which they had sexual intercourse? For example, memory failure and imprecision can compromise the accuracy of reported dates, with implications for STI/HIV research and control programs. The Project on Partner Dynamics (POPD) interviewed 536 young adults and 151 of their sexual partners. The 536 index participants were recruited from Los Angeles area community sites and interviewed every 4-months about perceptions and behaviors. At 8- and 12-months, index participants brought a partner for interview ("partner participant"). The 151 unique index-partner dyads were interviewed separately and jointly about partnership dates. Poisson regression models using generalized estimating equations (GEE) estimated the association between perceived partner non-monogamy (PPNM) and concurrency initiation (incidence). We also compared index and partner participants' reports of dates of first and last sex to estimate inter-partner agreement (IPA), and used linear regression to model the log of the differences. At 4-, 8-, and 12-month interviews, 4-month concurrency incidence was 8.5%, 10.6%, 17.8%, respectively. Participants with recent PPNM were more likely to initiate concurrency (crude risk ratio (RR)=4.6; 95%CI=3.0, 7.0; adjusted RR=4.0, 95%CI=2.6, 6.1). IPA (within 30 days) was low-to-moderate for first sex (43.1%), and high for last sex (94.5%). For both first and last sex dates and within each dyad: participants who were female (54.7% vs. 45.3% for first sex; 62.5% vs. 37.5% for last sex), had fewer sex partners (58.5% vs. 41.5% first sex; 54.8% vs. 45.2% last sex), or had greater commitment (56.3% vs. 43.7% first sex; 52.2% vs. 47.8% last sex) were in more agreement with joint dyad reports. PPNM and concurrency are associated, and at least in many cases, PPNM precedes concurrency imitation. Methods that increase reporting accuracy for partnership dates could improve concurrency research.