Post-treatment reactions to single-dose ivermectin (200 microg/kg) and albendazole (400 mg) were studied in a filarial endemic region of Mali. The prevalence of Wuchereria bancrofti in this region was 48.3% (69 of 143), and coinfection with Mansonella perstans was common (30 of 40, 75%). Microfilarial levels of M. perstans correlated positively with age (P = 0.006) and with W. bancrofti microfilarial levels (P = 0.006). Forty individuals (28 infected and 12 uninfected) were treated, with mild post-treatment reactions occurring in 35.7% (7 of 28) of the W. bancrofti-infected subjects. Reaction severity correlated with pretreatment W. bancrofti microfilarial levels (P = 0.001). There were no significant differences in the prevalence or severity of post-treatment reactions in those who were co-infected with M. perstans. It is concluded that co-infection with M. perstans does not significantly alter the post-treatment reaction profile to single-dose ivermectin/albendazole in W. bancrofti infection in this community, and that acute post-treatment reactions should not limit patient compliance in community-based programs to eliminate lymphatic filariasis.