The diethylcarbamazine (DEC) provocative day test has been widely used for daytime diagnosis of Wuchereria bancrofti infections in areas where microfilariae exhibit nocturnal periodicity. Since DEC is also the primary drug for treatment of bancroftian filariasis, we examined the long term effect of the test on microfilaraemia in 2 groups of individuals receiving either 100 mg of DEC (n = 51) or placebo (n = 20). The low dose of DEC had a significant therapeutic effect. One year after treatment, the geometric mean intensity of microfilaraemia was reduced by 86·1%, and 10·6% of the individuals were amicrofilaraemic. No significant reduction of microfilaraemia was observed in the placebo group. The results imply that the DEC provocative day test should not be used as a diagnostic tool in follow-up studies on microfilaraemias.