Immunological studies on 15 patients with serpiginous choroiditis gave no definite indications of the aetiology of the choroidal vascular lesion which appears to be the initial failure in this disorder. On the basis of the patients' history, recurrencies, and clinical and fluorescein angiographic features, it was suspected that the disorder was of inflammatory origin, probably vasculitis due to an abnormal immune response. To some extent this was supported by the laboratory findings. Of the histocompatibility antigens, HLA-B7 was found more frequently than expected in a Finnish population (54.5% versus 24.3%; P less than 0.05). Increased levels of antibacterial antibodies, ASO or ASTA, were found in eight patients and antiviral (herpes simplex) antibodies in two. One patient had earlier been treated due to positive syphilis serology, and one had an increased serum level of IgM as well as positive latex reaction in a dilution of 1:16. The serum concentration of complement component C3 was slightly decreased in three of the six patients studied and at the lower limit of the normal range in one. No manifestations suggesting systemic disease were found.