Iran is currently dealing with an outbreak of non-highly pathogenic avian influenza due to H9N2 sub-serotype in commercial layer flocks. The objective of this study was to evaluate some ethiological reasons about egg drops observed in naturally infected flocks following the first outbreak in July 1998. This study was carried out in a large multi aged egg layer farm possesing 40 flocks in Qazvin province. The serological antibody responses against avian influenza virus (AIV), Newcastle disease virus (NDV) and egg drop syndrom 76 (EDS76) virus were measeard by haemagglutination inhibition (HI) test between July 1998-1999. Comparison of HI antibodies titer of Amy, NDV and EDS76 was restricted to 10 flocks belonging to blocs A and B of farm. Both blocs were infected with Al at 12 weeks olds. Whereas bloc B was also involved with infectious ,laryngotracheaitis (ILT) after vaccination. The mortality in bloc B was 9.03% more than bloc A as compared at 26 weeks old, indicating that Al and ILT coinfection may increase Al mortality. An average of 9.64% egg drop was recorded only in bloc A flocks at 30 weeks old. There was a good correlation between decrease in HI antibody titer against AIV and decrease in egg production. Overall, it is suggested that decrease in egg production in naturally infected layer farms may be due to reemergence of field AIV isolates.