b Coccidioidomycosis (CM), a serious life-threatening fungal infection endemic to arid regions of the western United States and Mexico, can be challenging to diagnose in a timely manner. Commercially developed enzyme immunoassays (EIAs) (from Meridian Biosciences and Immuno-Mycologics [IMMY]) have provided faster, simpler means for serodiagnosis; however, independent evaluations have questioned EIA specificity, particularly IgM-positive/IgG-negative results. This study was conducted to evaluate EIA specificity among persons residing in Puerto Rico (n ‫؍‬ 534), where CM is not endemic (who were not likely to have been exposed to Coccidioides spp.), compared to blood bank donors residing in Arizona (n ‫؍‬ 1,218), where CM is endemic. Upon comparing serum reactivity between Puerto Rico and Arizona, the Meridian EIA showed a significant difference in IgG reactivity (0.37% versus 3.6%; P < 0.001) but not IgM reactivity (3.4% versus 2.4%; P ‫؍‬ 0.31). No IgM-/IgG-reactive sera were detected among sera from Puerto Rico, compared to 7 (0.57%) sera from Arizona. Similar results were observed using the IMMY EIA, although significantly (P ‫؍‬ 0.03) fewer IgM-reactive sera from Arizona were observed, compared to the Meridian EIA. EIA-reactive sera were also evaluated by immunodiffusion before and after 3-to 4-fold concentration of the sera. These results demonstrate that elevated IgG EIA reactivity is present in sera from healthy individuals in regions of endemicity and that IgM EIA reactivity observed in sera from individuals residing outside regions of endemicity is most likely nonspecific. Other criteria, including clinical and microbiological evaluations, should be taken into account when interpreting results from surveillance studies and other reporting measures. C occidioidomycosis (CM) is a serious life-threatening fungal infection endemic to arid regions of the western United States and Mexico (1, 2). Providing timely accurate diagnoses is important, particularly for high-risk patients who are immunosuppressed or of East Asian or African American ancestry (3, 4). Serodiagnosis is an important tool for the diagnosis of CM. However, the current serological gold standards of complement fixation (CF) and immunodiffusion (ID) are tedious and timeconsuming and require technical expertise of which few laboratories are capable. Due to its high throughput, ease of use, and increased sensitivity, enzyme immunoassay (EIA) is a diagnostic method that can be performed in smaller and less sophisticated laboratories that are closer to patients. As a result, EIA has become widely used for the diagnosis and surveillance of CM.A commercially available EIA (Premier Coccidioides EIA; Meridian Biosciences, Cincinnati, OH) for the separate detection of both IgG and IgM antibodies against Coccidioides has been in use for over 15 years. Initial reports described the assay sensitivity and specificity for the detection of IgG as 92% and 97% and those for the detection of IgM as 74% and 96%, respectively. Combining the results from the two as...