We evaluated, comparatively, the performance of female and male Triatoma infestans in the 5th instar of development, in xenodiagnosis for Chagas' disease. Xenodiagnosis were done in 40 patients with chronic infectans. For each diagnosis 20 nymphs female and 20 nymphs males were used, which were weighed immediately before and after the meal. Intestinal contents were examined about 20, 30, 60 and 90 days after the xenodiagnosis application. The females and males ingested the mean of 230 and 210 mg of blood, respectively. The results revealed positivity of 15 (37.5%) out of 40 xenodiagnosis, but any of the positive tests show 100% of the nymphs, female and/or male infected by T. cruzi. The females ingested significantly more blood than the males; this is coincidental with the higher female positivation to T. cruzi here observed. Our results suggest that the female nymphs seem able to increase the sensibility of the xenodiagnosis for chronic patients with Chagas' disease being necessary further studies are necessary to confirm this hypothesis.