Pathomorphological and biochemical investigations on liver and blood serum laying hens affected by the liver obesity syndrome were carried out. It was established that the mortality due to the liver obesity syndrome varies within the range of 3.1 and 3.7% for the entire period of exploitation. A rise in mortality is observed in case fodder mixtures with higher peroxide and aldehyde number are prepared. Besides the typical changes in the liver, the pathologo-anatomical investigation established varying in its expression duodenitis of rupture of the liver and hemorrhage. In hens suffering from advanced liver obesity an increased content of total protein in the blood serum was observed. The relative and absolute content of prealbumens and albumens was also higher, while the content of globulins was relatively lower. The content of beta-lipoproteins was raised and total lipids in the blood serum were considerably increased. The investigation on total lipids and lipid fractions in the liver established a correlation between the extent of obesity and the content of total lipids. A trend toward increasing the total and particularly the esterificated holesterin was evident in affected birds. The chemical investigation of various lots of fodder mixtures established often cases of rancid fats, which was manifested by high values of the peroxide and aldehyde number. The aminoacid composition of fodder also varied too much. It is assumed that besides the genetic control of liver obesity rancid fats and insufficient content of essential amino acids in the fodder mixtures also lead to an increased mortality percentage in the affected birds.