Hens laying eggs with egg-pole shell defects (EPS) were examined in a clinical prospective study. 86 hens with EPS and 72 hens without EPS from three flocks were selected for this study. It could be proven serologically that hens with EPS had significant (p < 0.05) higher titers against Mycoplasma (M.) synoviae then hens without EPS. PCR tested cloacal swabs for M. synoviae were more frequently positive from hens with EPS (87%; n=72) then from hens without EPS (18%; n=13). Furthermore, M. synoviae could be cultivated from the oviduct of five hens with EPS. Additionally, M. synoviae-DNA was detectable in the albumen of nearly all eggs with EPS (n=48; 98%), contrary to the eggs without EPS (n=11; 26%). Ultrastructural investigation revealed that eggs with EPS showed considerable differences of the egg shell structure as well as the cross section dimension according to eggs without EPS. Due to the significantly more frequent detection of M. synoviae-DNA from the cloaca of chickens with EPS, is an involvement of M. synoviae in laying of EPS in the surveyed herds likely. Further infection experiments with the isolated M. synoviae were not perfomed, therefore about the causal pathogenic role of M. synoviae in the development of eggs with EPS in the surveyed herds can only be speculated.