Treatment of subperiodic bancroftian filariasis, which is endemic in Samoa, with diethylcarbamazine citrate (DEC-C) in single doses of 4 mg/kg, 6 mg/kg, and 8 mg/kg body weight was evaluated using the nuclepore filtration method (with 1 ml blood) and compared in terms of efficacy against the microfilariae (mf) and side-reactions produced. The 6 mg/kg single-dose treatment assessed at six months showed that the effect of DEC-C to eliminate microfilariae was closely associated with the pre-treatment microfilarial level. The treatment cured nearly 60% of the low-density carriers with </=20 mf/ml but only about 10% of the carriers with >/=501 mf/ml. However, the percentage decrease in the microfilarial count, which averaged 89.3%, did not seem to differ greatly according to the level of the pre-treatment count. The age group 20-29 years showed a poorer response to the treatment compared with the other age groups. When the different dosage regimens (4 mg/kg, 6 mg/kg and 8 mg/kg) were compared at 6 and 12 months after treatment, the 6 mg/kg regimen was found to be more effective than the 4 mg/kg regimen in reducing the microfilarial count, and it produced fewer adverse reactions than the 8 mg/kg regimen. The comparison between the annual single-dose treatment at 6 mg/kg and the six-monthly two doses/year treatment at the same dosage (total 12 mg/kg/year) showed that the latter had little advantage over the former, thus indicating the effectiveness of the single-dose treatment for longer than six months.