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Between November 2005 and March 2006, a total of 253 poultry flocks in the Gyeonggi-do of Korea were examined for seroprevalence against avian influenza (AI) using a hemagglutination inhibition (HI) test and an agar gel precipitation test. No low pathogenic avian influenza (LPAI) virus was isolated from 47 seropositive flocks that lacked clinical signs during sampling. The unadjusted percentage of seroprevalence rates of layer and broiler flocks were not significantly different, i.e., 26% (25/96) and 23% (22/97), respectively. The HI titer of the layers (mean = 89) was higher than the broilers (mean = 36; p < 0.001). A cross-sectional study was conducted for the seroprevalence of LPAI in the layers. Of 7 risk factors, farms employing one or more workers had a higher seropositive prevalence as compared to farms without hired employees (adjusted prevalence OR = 11.5, p = 0.031). Layer flocks older than 400 d had higher seropositivity than flocks younger than 300 d (OR = 4.9, p = 0.017). The farmers recognized at least one of the clinical signs in seropositive flocks, such as decreased egg production, respiratory syndromes, and increased mortality (OR = 2.3, p = 0.082). In a matched case-control study, 20 pairs of case and control flocks matched for type of flock, hired employees, age, and flock size were compared. Frequent cleansing with disinfectants was associated with a decreased risk of seropositivity (OR = 0.2, p = 0.022). Although there was a low statistical association, using a foot disinfectant when entering the building led to a decreased rate of seropositivity (OR = 0.3, p = 0.105).