Analysis included data from the SYMLEK data base on 277 306 control milkings of cows that first calved in 2001 and were used in 2001-2006 in the Pomerania and Kujawy regions. Using analysis of variance, the energy content of milk was analysed by way of the least squares method according to the following factors: period of lactation, age of cows, season of the year, yield from control milking, somatic cell count per ml milk, level of herd productivity, and breed. The population studied was characterized by an average milk yield of 17.32 kg with 4.37% fat, 3.40% protein, 4.73% lactose and 13.18% solids content. Depending on the formula used, the average energy content of milk ranged from 788.65 to 719.27 kcal/kg. Milk energy content decreased in the initial stage of lactation and increased consistently from 60 days of lactation. Analysis of the effect of cows’ age on the energy content of milk showed that it peaked at the second and third lactation, and decreased gradually in the next lactations. Season of the year had a significant effect on the energy content of milk. The highest energy content (803.40 and 732.48 kcal/kg) was found in the milk obtained from December to February and the lowest (762.86 and 696.54 kcal/kg) in the milk obtained from June to August. Milk energy content decreased with increasing daily milk yield. The milk containing up to 100 000 somatic cells per ml, which is evidence of very good udder health, had the lowest energy content. Of all the breeds analysed, the milk of Jersey cows showed the highest energy content.