We compare estimates of multiple and concurrent sexual partnerships from Demographic and Health Surveys (DHS) with comparable Population Services International (PSI) surveys in four African countries (Kenya, Lesotho, Uganda, Zambia). DHS data produce significantly lower estimates of all indicators for both sexes in all countries. PSI estimates of multiple partnerships are 1.7 times higher [1.4 for men (M), 3.0 for women (W)], cumulative prevalence of concurrency is 2.4 times higher (2.2 M, 2.7 W), the point prevalence of concurrency is 3.5 times higher (3.5 M, 3.3 W), and the fraction of multi-partnered persons who report concurrency last year is 1.4 times higher (1.6 M, 0.9 W). These findings provide strong empirical evidence that DHS surveys systematically underestimate levels of multiple and concurrent partnerships. The underestimates will contaminate both empirical analyses of the link between sexual behavior and HIV infection, and theoretical models for combination prevention that use these data for inputs.