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 paper examines variation in both partner concurrency and individual concurrency among African men in Cape Town, South Africa, and assesses the relationship between different types of concurrent partnerships and STDs.\nMethods: Longitudinal data from sexual partner history tables were used to form measures of concurrency, and the type of partner (main vs. non-main) and degree of condom use (consistent vs. inconsistent) associated with these concurrent relationships. Cross-sectional data from a self-administered module were also employed to assess the number of partners men had had concurrently and duration of individual concurrency. The association between the concurrency measures and self-reported STD status was assessed using probit regression models.\nResults: Substantial differences between concurrent sexual partnerships were observed and these variations were associated with different disease risk. Men had a significantly greater chance of having an STD when partner concurrency was associated with main partners and inconsistent condom-use, and when individual concurrency involved a greater number of partners concurrently and long-term partnerships.\nConclusion: Partnership dynamics must be taken into account in studies assessing the role of concurrency in STD transmission and in STD prevention programs. Reducing concurrency (either in terms of number of partners or duration) and/or encouraging consistent condom use in concurrent partnerships would help reduce STD infections.\n