Several animal studies have shown a protective effect of helminth infections against type-1 diabetes mellitus (T1DM). However, epidemiologic studies demonstrating this protective relationship with T1DM are largely lacking, although an inverse correlation between the prevalence of lymphatic filariasis (LF) and prevalence of allergies and autoimmunity has been shown. A cross-sectional study was undertaken in southern India to assess the baseline prevalence of seropositivity of LF among persons with T1DM (n = 200) and normal glucose tolerant (NGT) persons (n = 562). The prevalence of LF was 0% among persons with T1DM and 2.6% among NGT persons (P = 0.026). The percentage of persons who were positive for filarial antigen-specific IgG4 (but not antigen-specific IgG) was also significantly lower in persons with T1DM (2%) compared with NGT persons (28%) (P < 0.001). Thus, there appears to be a striking inverse relationship between the prevalence of LF and T1DM in southern India.