Strongyloidiasis is a chronic infection that may result in significant morbidity; however, diagnosis and management remain problematic. The objective of this study was to 1) evaluate the demographic, clinical, and laboratory features of 76 consecutive individuals who had Strongyloides stercoralis larvae identified in their fecal specimens; 2) determine the sensitivity of the Centers for Disease Control and Prevention (CDC) enzyme immunoassay (EIA) for detecting antibodies to Strongyloides in those with confirmed infection; and 3) assess the serologic responses and changes in eosinophil counts following treatment. Most (96%) cases occurred in immigrants, but some patients had immigrated as long as 40 years earlier. The CDC Strongyloides EIA had a sensitivity of 94.6% (95% confidence interval = 92.0-97.2%) in this patient population with proven infection. Serologic and eosinophil counts decreased after therapy, suggesting that they may be useful markers of treatment success.