Characteristics of variation of vegetation period duration at the Agrometeorological Observatory at Felin constitutes an attempt to confirm scientific reports on increasing dispersion of vegetation duration over many years and on its tendency to terminate earlier in the SE Poland. A series of observations of mean daily air temperatures from a many-year period of 1951–2000 was analyzed. Using the method of Huculak and Makowiec, the dates of passing the threshold value of air temperature of 5°C were determined. This threshold value fences the start – in spring, and the end – in autumn of the vegetation period. The applied method explicitly determines these dates and eliminates incomparability of the results. The data obtained were analyzed statistically and classified into 5 classes, basing on standard deviation values. The number of years in an individual class allow us to calculate the occurrence frequency of vegetation periods of a specific type, attributed to a given class. As a result of the division of the last 50-year period into successive 10-year intervals it appeared that the last two decades are characterized as follow: the period 1981–1990 shows most stable dates of the start and the end of vegetation period, while the last decade, i.e. 1991–2000 has wide amplitude of the beginning of the season and the highest of the ending (45 days). Standard deviation for the seasons end in this decade equals to 14.8 days and is considerably higher both in relation to the preceding decade (8.4 days) and to the entire 50-year period (11.4 days). The trend line confirms a distinct tendency to earlier ending of the vegetation season in this part of the country. Also, the duration time of the vegetation period stands for significant differentiation over many years. It can be either shorter by 41 days as compared to average duration time (215 days) or longer by 36 days. However, the classification adopted (where the anomaly criterion was considered xsr = 2δ) proved that only the year 1997 was exceptionally short, as far as the seasons duration time is concerned. On the other hand, the year long did not occur at all. Most vegetation periods (76%) represented average duration time. Only 10% can be classed as long and 12% as short. This seems to prove the differentiation typical transition climate of Poland. It is worthy to note that possible anomalous phenomena should be analyzed for more measurement sites, including spatial characteristics.