BACKGROUND: Toxocariasis is one of the major causes of peripheral blood eosinophilia and provokes eosinophilic infiltration in the internal organ. Extended studies of the incidence and clinical manifestations of toxocariasis have been rare. OBJECTIVE: The purpose is to evaluate the prevalence and clinical characteristics of toxocariasis in patients with unknown eosinophilia, and to analyze the efficacy of toxocara ELISA. METHOD: Patients with presenting with peripheral blood eosinophilia (>500/microliter or > or =10% of WBC) from Oct. 2001 through Jan. 2005 were enrolled. After checking the drug histories and the presence of allergic diseases or parasite infections, specific serum IgG Ab to Toxocara canis larval antigen was measured by ELISA. Liver and lung involvement was also evaluated. RESULT: One-hundred and three patients (81 men, 22 women) were evaluated. Seventy patients (68.0%) were diagnosed with toxocariasis. The patients who had a history of eating raw liver had a higher incidence, and the patients with liver involvement had higher serum ECP values. The improvement rate was not different between treated and untreated patients. The mean onset of improvement was 12 months. CONCLUSION: The prevalence of toxocariasis is high in patients with unknown eosinophilia. The toxocara ELISA is essential for evaluation of the cause of unknown eosinophilia.