This case-control study was carried out to investigate whether eosinophilia is a common finding in parasitic diseases and to determine the prevalence of the parasitic diseases in definitive groups. Toxocariasis and fasciolosis were investigated in patients with eosinophilia, patients with non-eosinophilia, and healthy controls. All cases were analyzed for antibodies against Toxocara canis and Fasciola hepatica by modified enzyme-linked immunosorbent assay (ELISA) employing, as antigen, excretory/secretory products of Toxocara and Fasciola (ES-ELISA). Seropositivity was determined in 39 (29.1%) and 12 (8.9%) toxocariasis and fasciolosis patients with eosinophilia respectively. The seropositivity was found in 26 (19.4%) and 5 (3.7%) toxocariasis and fasciolosis patients without eosinophilia respectively. The seropositivity was found in 13 (15.5%) and 1 (1.2%) toxocariasis and fasciolosis patients in the control group. The total seropositivity in both toxocariasis and fasciolosis in the eosinophilic group (8.5%) increased significantly compared to patients without eosinophilia (23.1%) and healthy controls (6.6%) (chi(2) :6.343, p<0.05 and chi(2): 10.293, p<0.01). Eosinophilia correlated with antibodies against Toxocara and F hepatica. The results suggest that serological methods can be used for diagnosis of parasitic infections in tissue. Thus, the actual prevalence of tissue parasites can be revealed particularly in developing and emerging countries.