Core population groups play an important role in the spread of sexually transmitted diseases. Subjects in a core group may change their behavior over time and “migrate” to the noncore. The authors examined the effects of such migration on the prevalence of gonorrhea, chlamydia, and human immunodeficiency virus (HIV) using a mathematical model. The size of the core and the migration rate from the core to the noncore were estimated from population-based sexual survey data on 8,445 Norwegians collected in 1987 and 1992. Sixty-four percent of the sample was considered without risk of contracting a sexually transmitted disease. The core group made up 2.5% of the remaining sample. The migration rate from the core was estimated at 12% per year. The three types of infections analyzed exemplify three different patterns of the effect of migration on infection prevalence in the core/noncore groups: gonorrhea = no effect–no effect, chlamydia = no effect/increase, and HIV = decreas/increase. Migration affects the basic reproductive ratio of diseases with a long infectious period more than that of diseases with a short infectious period. For HIV, this means that the later stages of infection contribute less to the basic reproductive ratio in the presence of migration. The results are qualitative and show that detailed knowledge about mixing, migration, transmission rates, and duration of infectiousness is necessary to make accurate predictions.