This study assessed Wuchereria bancrofti-infected patients who received diethylcarbamazine (DEC) to determined if drug levels were comparable between individuals who continued to harbor circulating microfilariae and those whose blood became clear of parasites, between those with high and low microfilaria counts, and between infected and noninfected persons. Haitian volunteers undergoing treatment for W. bancrofti and two nonendemic controls were enrolled. DEC levels in serum and urine samples were determined using a gas chromatographic method. No correlation was found between pre- and posttreatment microfilarial levels and drug levels. Drug levels were comparable in persons with or without residual microfilaria and in infected and uninfected persons. These results indicate that incomplete drug regimens, differences in serum drug levels, and inadequate drug dosage are not the primary causes of treatment failure and suggest there is a degree of parasite tolerance for DEC.