Concurrent partnerships may cause more rapid spread of HIV in a population. We examined how the prevalence of parallel relationships changed among men and women aged 15-49 in Zambia from 1998 to 2003 using data collected during the Sexual Behavior Surveys 1998, 2000, and 2003. Predictors of concurrent partnerships among men were studied by univariate and multivariate regression analyses. Thirteen percent of rural and 8% of urban men reported more than one ongoing relationship in 1998, and these proportions declined to 8% and 6%, respectively in 2003. The proportion of women reporting concurrent relationships was 0-2%. The most important predictors of concurrency were early sexual debut, being married, early marriage and absence from home. The reduction in concurrent sexual partnerships is consistent with reductions in other sexual risk behaviors found in other studies and may have contributed to the recently observed decline in HIV prevalence in Zambia.