Gizzard erosion and ulceration (GEU) caused by fowl aviadenovirus serotype 1 (FAdV-1) of the species Fowl aviadenovirus A (FAdV-A) represents an economically important problem in poultry production. The disease affects mostly young chicken broilers or layers before production. In this study, an unusual GEU outbreak in a flock of laying hens at 38 weeks of age is described. The affected flock showed elevated mortality rates, with the highest number of dead birds appearing between the 39th and 40th week of life, with a subsequent reduction in laying performance and decreased total egg weight. Post-mortem examination showed the presence of erosion in multiple areas of the gizzard, with wall perforation in the proximity of the interventriculus. FAdV antibodies were detected in all examined sera with an ELISA assay. The virus was isolated from pathologically altered gizzards. PCR, subsequent sequencing and phylogenetic analysis of the partial hexon gene confirmed the presence of FAdV-A DNA. To investigate the molecular background of FAdV-A which causes GEU in adult hens, whole genome sequencing was performed on two FAdV-A strains - strain W-15, obtained from the outbreak described in this study and strain 61/11z, isolated from a GEU outbreak in 3-week-old broiler chickens in 2011. The genome size of FAdV-A W-15 is 43,849 bp. Genome sequence and genome organization resembles those of the reference, apathogenic CELO strain and the newly sequenced GEU strain, 61/11z. Most amino acid changes, between CELO and GEU strains, were observed in ORF0, ORF1, ORF14, IVa2, polymerase, pIIIa, penton base and fiber-2. Analysis conducted on the translated ORFs revealed that W-15 and 61/11z are nearly identical, with the highest rate of amino acid mutations in pTP, 100K, ORF9 and ORF10. In this study, the occurrence of GEU, caused by FAdV-1 infection, in adult layer chickens and the effects of such infection on egg production parameters are described in detail. Moreover, the whole genome sequences of two pathogenic, GEU inducing FAdV-A strains have been provided and characterized for the first time, which in the future will help to pinpoint the viral factors involved in pathogenicity.