Background Cockroaches have been demonstrated to be an etiologic factor in allergic diseases. Further, sensitivity to cockroach places patients with asthma at risk for exacerbations that require emergency medical care. Objective This study compared the differences in allergenic components between German cockroach whole body and German cockroach fecal extracts (GWBE and GFE). Methods Patients with asthma and/or allergic rhinitis were skin prick tested with German cockroach extract (Bayer Corporation, West Haven, CT). Serum specimens from these patients, 25 with positive skin tests and 8 with negative tests, were used for the ELISA and immunoblot experiments. Results By ELISA, 72% (18 of 25) and 60% (15 of 25) of positive responders' sera showed IgE antibodies to GWBE and GFE, respectively, and the IgE levels to GWBE were highly correlated with those to GFE ( r = .84, P < .01). In inhibition ELISA experiments, extensive cross-reactivity was observed between GWBE and GFE, slight cross-reactivity between GWBE and Dermatophagoides farinae, and no cross-reactivity between GFE and D. farinae. The two-site monoclonal antibody ELISA detected more of the German cockroach major allergens in GFE compared with GWBE; 6.2 times (2420 vs 390 U/mL) for Bla g 1 and 3 times (15.32 vs 5.07 μg/mL) for Bla g 2. In the immunoblot comparison of patients' sera, the IgE antibodies binding to GWBE were apparently different from those binding to GFE in all the positive responders' sera; eg, 50% or more of the 25 positive responders' sera reacted to 43- to 67-kDa proteins in GWBE and to 28- to 30-kDa proteins in GFE, respectively. No IgE antibodies bound to components in GWBE and GFE in the 8 negative responders' sera. Conclusions There are major differences between the allergenic components of GWBE and GFE. Based on the amounts of major allergens (Bla g 1, Bla g 2), German cockroach feces are a more important source of allergen than the whole body in respiratory allergic diseases.