We examined the periodicity and intravascular distribution of Wuchereria bancrofti microfilariae (rnf) and determined the effect of these parasite properties on the accuracy of blood filming and filtration methods for diagnosis of bancroftian filariasis in the endemic area of Recife, Brazil. Microfilariae in both venous and capillary blood exhibited a nocturnal periodicity pattern with a relatively high amplitude. Overall, capillary blood contained -1.25 times the number of rnf present at the same time in the same volume of venous blood. However, the ratio of rnf present in capillary and venous blood varied over a z4-hour period, so that the fewest mf were present in the capillary bed of the skin at the time when biting activity of the local Culex vector is the lowest. Twenty or 6 0 p1 blood films did not reliably detect carriers with fewer than IOO or 60 rnf/ml venous blood, respectively, and were thus inadequate for the identification of low density rnf carriers. In contrast, all carriers with > I mW2o or 60 pl blood smear at night could be identified during daytime hours by filtration of I ml venous blood keywords Wuchereria bancrofti, periodicity, distribution, capillary blood, venous blood, thick smear. filtration