The study investigated the fatty acid composition (fatty acids, FA) of 19 samples of butter originating from the north–eastern region of Poland throughout the year. The annual variability was observed in the FA composition of butter. A higher content of short and medium chain saturated FA (SCFA) was determined in the samples from the winter season (12.6-13.4%) and lower one in the summer samples (11.2-12.1%). The major FA of the saturated long chain acids (LCFA) of butter were: palmitic C16:0: (25.7-34.6%) and myristic C14:0: (9.7-11.9%), their higher contents were found in the samples from the winter and early-spring production. Butters produced in the summer period were characterised by a higher content of oleic acid C18:1 9c (20.6-23.1%) belonging to monounsaturated FA (MUFA) as compared to the samples produced in the winter and spring (18.3-21.4%). Contents of FA belonging to polyunsaturated fatty acids (PUFA) were: 1.0-1.5% for linoleic C18:2 9c 12c (LA) and linolenic acids C18:3 9c 12c 15c (ALA), which were subjected to seasonal variation in the range of 0.1-1.0%. The content of predominant trans isomer in ruminant fats, i.e. vaccenic acid C18: 1 11t (VA), resulting from “biohydrogentaion” of the PUFA (LA and ALA) in the rumen, has been strictly connected with the season of production: a higher content of VA was determined in butter samples from the summer and autumn (1.9-3.8%) and lower one in the samples produced in the winter and spring (0.8-1.3%). Similarly, seasonal variation was also observed in the content of conjugated linoleic acid C18: 2 9c11t (CLA). Its significant content – 580 mg CLA /100 g of product on average – was determined in butter samples produced in the summer and autumn season in the north-eastern region of Poland.