Clinical and biological evaluations were carried out on 84 Congolese patients with parasitologically confirmed Loa loa filariasis (without concurrent infection with other filariae) and on 98 controls without filariasis. On the patients, 72 presented with microfilaremia; another 12 with negative blood tests were seen towards the end of an episode of subconjunctival migration of the adult worm. The incidence and severity of the clinical signs depended upon the method of recruitment. The 3 most common signs were pruritus and edema (both occurring in successive acute episodes affecting mainly the hands and forearms) and subconjunctival migration of adult filariae. Papulovesicular eruptions were located mainly on the arms. Headaches and arthralgia were noted more frequently than in the controls. No relation was found between the ABO blood groups and loiasis. Eosinophilia (higher in patients with symptoms) and raised serum IgE levels were found in nearly all patients and were strongly marked in approximately 66%. A positive correlation was observed between these 2 parameters. Fluorescent antibody levels (adult filaria Dipetalonema viteae antigen) were comparatively low in patients with microfilaremia.