IMPORTANCE Bullous pemphigoid (BP) is by far the most frequent autoimmune blistering disease. The presence of IgE autoantibodies against the transmembrane protein BP antigen 2 (BP180, type XVII collagen) has previously been reported in 22%to 100% of BP serum samples, and the pathogenic relevance of anti-BP180 IgE has been suggested in various experimental models and by the successful use of omalizumab in individual patients with BP. OBJECTIVES To determine the rate of anti-BP180-reactive IgE in BP, to evaluate the diagnostic relevance of anti-BP180 IgE in BP, and to correlate anti-BP180 IgE with disease activity and the clinical phenotype of patients with BP. DESIGN, SETTING, AND PARTICIPANTS This case-control cohort study examined 3 groups of patients with BP. Sixty-five patients with BP underwent an enzyme-linked immunosorbent assay for IgE antibodies against the 16th noncollagenous domain of BP180 (NC16A); 52 consecutive patients with BP underwent clinical evaluation with the Bullous Pemphigoid Disease Activity Index (BPDAI); and 36 patients with BP without anti-BP180 NC16A IgG reactivity underwent evaluation of the diagnostic importance of serum anti-BP180 IgE. In addition, 49 age-matched control individuals with noninflammatory dermatoses, 127 controls undergoing allergy testing for IgE levels, and 30 controls with pemphigus vulgaris or pemphigus foliaceus were included for comparison. Patients were seen at a university clinic from January 1, 2008, to July 31, 2014. MAIN OUTCOMES AND MEASURES Serum anti-BP180 NC16A IgE and IgG levels and BPDAI scores. RESULTS Of 117 patients with BP (69 women and 48 men), anti-BP180 NC16A serum IgE was detected in 47 (40.2%) and correlated with disease activity as measured by total BPDAI (r = 0.918; P = .06). An intraindividual correlation of anti-BP180 NC16A serum levels with the total BPDAI was observed during the course of the disease in 10 randomly selected patients with BP (r = 0.983; P = .003). Although no association of circulating BP180 NC16A IgE antibodies with urticarial or erythematous lesions was observed (r = 0.481; P = .31), the presence of IgG anti-BP180 NC16A antibodies was associated with the occurrence of erosions and blisters (r = 0.985; P = .006) but not urticarial and erythematous lesions (r = 0.632; P = .23). Assaying for anti-BP180 IgE increased the diagnostic sensitivity by only 2.2%(1 of 46 serum samples) when combined with the IgG anti-BP180 enzyme-linked immunosorbent assay. CONCLUSIONS AND RELEVANCE Although detection of serum anti-BP180 IgE is not of diagnostic importance, itmay be relevant for therapeutic decisions (eg, the use of anti-IgE treatment). The correlation of serum anti-BP180 NC16A IgE levels with disease activity in patients with BP supports the notion that anti-BP180 IgE is of pathogenic relevance. Our observation that IgG anti-BP180 antibodies are related to the occurrence of blisters and erosions may encourage further studies on the association of fine autoantibody reactivities with clinical features of BP.