Studies on serum-mediated adherence of leucocytes to microfilariae of Wuchereria bancrofti in vitro were carried out in an area of Tanzania endemic for filarial infections. Patients were divided into age-matched groups according to parasitological and clinical conditions. No adherence was seen with serum from microfilariae-positive patients (with or without clinical disease). A slightly, but not statistically significantly, higher prevalence of adherence-positive sera was found among hydrocele patients (27%) and lymphangitis/elephantiasis patients (33%) than among endemic control patients (20%). These three positive groups exhibited a marked difference in age distribution of prevalence of positivity, with very high prevalences found among young hydrocele patients (48%) and old lymphangitis/elephantiasis patients (63%). The hydrocele fluid from hydrocele patients with adherence-positive serum also mediated adherence.