Eggshell abnormalities were seen in the apex of eggs in two of three flocks of multi-age, Hy-Line layer chickens housed on a farm in Northern Italy. Approximately 1.3% to 1.8% of eggs in one flock were affected, amounting to 300–400 eggs per day; the abnormalities resulted in a great deal of breakage and spoilage of healthy eggs. The mean weight of eggs was also reduced. Egg abnormalities in a second flock were less severe. Mycoplasma synoviae was detected in birds from both of the affected flocks by serologic, cultural, and molecular techniques, but not in a third, adjacent flock where no eggshell abnormalities were seen. Treatment with tylosin, administered in the drinking water for 5 days, resulted in an immediate improvement of eggshell quality and egg weight. There was no evidence of infectious bronchitis virus in the flocks.