The article investigates the longevity of adult men in the magnate families of the Grand Duchy of Lithuania in the 16th-18th c. and the consequences of their age at death for their relatives. The research concerned representatives of three magnate houses: the Radziwiłł, Ogiński and Chodkiewicz families. The database was assembled on the basis of published biographies, articles, family monographs, genealogical tables and manuscripts from Polish and Lithuanian archives. Data was gathered on 175 men: 85 of the Radziwiłł, 56 of the Ogiński and 34 of the Chodkiewicz family. In order to describe the consequences of a magnate’s death it is important to consider the average marrying age. The average was calculated for 104 men; taking into consideration first and subsequent marriages it was almost 32 years in the period researched. For first marriages the average was 27, while for subsequent marriages ca 42. It is also important to calculate the average length of marriage. In the 16th-18th c. it was 14.5 years, with first marriages lasting 16 years and subsequent ones 12 years on the average. The average for first marriages with children was 17 and for subsequent marriages with children – 14 years. The average death age, calculated for 96 adults, including married and unmarried men, was just under 50 for the whole period in question (in the 17th c. it was 46, while in the 18th c. it was over 50). For married men (79 persons) the average was 53, and for married men with children (54 persons) – 54 years. Thus, a man in a magnate family was married for the first time at the age of 27 on the average, and a man with progeny lived to 56 years on the average. The first marriage having children lasted 17 years on the average, and subsequent marriages with children - even shorter. In magnate families 70% of children survived, and these were often not first children. This indicates that at the time of the father’s death children were often still under age. This often meant that children were taken care of by the father’s relatives, especially if the widow remarried. In this case mothers usually lost custody of their children and their emotional bond with them was broken. Hence, one of the consequences of a magnate’s death was that the widow had to choose between remarrying and keeping custody of her children and their property. The latter choice meant no further procreation for many young women. A man in a magnate family often decided to marry after his father’s death, whether following or disregarding his earlier suggestions as to the choice of the wife. The death of a childless magnate lead to the extinction of the family or one of its lines, which resulted in transferring property to other families or lines. The research indicates that the average lifespan was longest in the Ogiński family: for the period in question it was 57 years both in general and for married men with children. To compare, the married Radziwiłłs with children lived for 53.5 years on the average. It can be supposed that the better health condition of the Ogińskis, especially in the 17th c., might have resulted from their late advancement to the country’s political elite (in the second half of the 17th c.), which meant that the circle from they chose their life partners was limited to the narrow sphere of magnate families relatively late.