BACKGROUND Most studies that assess the relationship between concurrent sexual partnerships and sexually transmitted diseases (STDs) use dichotomous measures of whether concurrency was reported or not. However, different forms of concurrency have different degrees of associated risk for disease transmission, and this should be considered. This article examines variation in both individual concurrency and partner concurrency among African men in Cape Town, South Africa, and assesses the relationship between different types of concurrent partnerships and STDs. METHODS Longitudinal data from sexual partner history tables are used to form measures of concurrency and the type of partner (main vs. nonmain) and degree of condom use (consistent vs. inconsistent) associated with these concurrent relationships. Cross-sectional data from a self-administered module are also used to assess the number of partners men have had concurrently and duration of individual concurrency. Probit regression models assess the association between the partner concurrency measures and self-reported STD history. RESULTS Substantial differences between concurrent sexual partnerships were observed and these variations were associated with different disease risk. Men had a greater chance of reporting an STD when partner concurrency was associated with main partners and inconsistent condom use. CONCLUSION Partnership dynamics must be taken into account in studies assessing the role of concurrency in STD transmission.