Iron deficiency anemia (IDA) is a frequent health problem. Gut parasites such as N. americanus and A. duodenale are known to cause blood loss, but the role of Blastocystis hominis is uncertain. In this study, 212 patients (193 female, 19 male, mean age 41 SD 15 yrs) with IDA were enrolled and 90 persons without IDA (78 female, 12 male, mean age 45 SD 17 yrs). Microscopic examination of stools for B. hominis using the native lugol method was done three times on each subject. If any specimen contained five or more cysts per x400 field, the person was considered positive. B. hominis was found in 48 out of 212 subjects with IDA (22.6%) and in five of 90 (5.6%) subjects without IDA. This difference is highly statistically significant (P < 0.001). Few subjects had other gut parasites and there was no statistical difference in the ir frequencies between IDA and non-IDA subjects. Blastocystis hominis may play a role in the development of IDA either on its own or in conjunction with some other agent.