The proportion of cases of genital ulcer disease attributable to herpes simplex virus type 2 (HSV-2) appears to be increasing in sub-Saharan Africa. To assess the contributions of HIV disease and behavioral response to the HIV epidemic to the increasing proportion of genital ulcer disease (GUD) attributable to HSV-2 in sub-Saharan Africa. Simulations of the transmission dynamics of ulcerative sexually transmitted diseases (STDs) and HIV with use of the model STDSIM. In simulations, 28% of GUD was caused by HSV-2 before a severe HIV epidemic. If HIV disease was assumed to double the duration and frequency of HSV-2 recurrences, this proportion rose to 35% by year 2000. If stronger effects of HIV were assumed, this proportion rose further, but because of increased HSV-2 transmission this would shift the peak in HSV-2 seroprevalence to an unrealistically young age. A simulated 25% reduction in partner-change rates increased the proportion of GUD caused by HSV-2 to 56%, following relatively large decreases in chancroid and syphilis. Behavioral change may make an important contribution to relative increases in genital herpes.