In order to estimate the biothermic conditions of north-east Poland, Błażejczyk’s sensible temperature index STI was applied, which is based on the analysis of human heat balance. STI value was determined using the simplified version of the model based on the following term data from 12 o’clock UTC: air temperature, relative air humidity, steam pressure, wind speed, and cloudiness by low (C_L) and middle (C_M) clouds. Data were taken from 10 meteorological stations in north-east Poland, and the period of the study was years 1966–2000. Sensible temperature index STI fell within the range of below -38°C to over 55°C. To the particular values of the STI index, Błażejczyk attributed heat sensations in a seven-degree scale of subjective heat sensations – from "very cold" to "very hot". Mean STI values and the frequency of the occurrence of the particular heat sensations were presented for months, seasons, and a year. Time trends of sensible temperature STI were determined for four seasons and a year, and a spatial diversification of this index in north-east Poland was carried out. Sensible temperature STI showed a statistically significant increase in the period of 35 years in all seasons only in Suwałki, and at the remaining stations, a significant increasing trend of this temperature occurs during spring and winter. The highest sensible temperature increase, from 0.57°C in Płock to 1.99°C in Suwałki fell in the spring, except the station in Ostrołęka where in that season, a negative and statistically insignificant tendency was noted. Spatial picture of the distribution of sensible temperature STI shows an increase in its values from the north towards the south of the studied area in all the seasons of the year except winter, in which a decrease in STI values takes a direction from SW to NE.