A case-control study was performed in Martinique, French West Indies, comparing 66 anti-p24 antibody carriers to 91 seronegative subjects for HTLV-I, matched for age and place of residence. The aim of our study was to identify factors associated with HTLV-I infection and to observe whether clinical examination and biological measurements would reveal any abnormalities among the seropositive subjects. We observed a predominance of females among seropositive subjects (74% compared to 59%, p less than 0.05), and a greater risk due to earlier blood transfusions (p less than 0.001). This survey revealed important differences between cases and controls regarding socioeconomic factors: cases had fewer luxuries or advantages (i.e. bathroom, toilets, refrigerator, telephone, p less than 0.01), were more corpulent (p less than 0.05), and more often widowed, divorced or separated (p less than 0.01) than the controls. Although the differences were not significant, the seropositive donors seemed to be less educated, and were from a lower socioprofessional class than the seronegative donors. With regard to clinical symptoms (infections, adenopathies, splenomegaly, hepatomegaly) and biological parameters (blood count; T-cell subsets, electrophoresis of protids, immunoglobulins, calcemia, antischistosomal antibody), seropositive subjects appeared to be healthy; no parameters, except for alpha 1 globulin (p less than 0.05) and monocytes (p less than 0.05), were found to be correlated with seropositivity; but these two parameters remained within their normal ranges. This study confirms blood transfusion as a risk factor. It underscored the importance of socioeconomic factors for seropositivity.