The approach to the health-disease problem in the family requires a knowledge of demographic features. We propose a demographic classification based on the nuclear family. In addition to the extensive and single-parent families, the lack of family and the familial equivalents we subdivide the nuclear family depending on whether it has close relatives or not, the number of children, the existence of extended family or not, with relatives or aggregates, and the binuclear families or those of divorced people. On this basis, we have evaluated the demographic distribution of 917 families from the basic health areas of Albaicín and Cartuja in Granada, the Valle de Jaén, and Telde in Las Palmas. We have found the predominance of the nuclear family (76.8%) over the extensive (5.2%), as well as the relevance of the single-parent (8.9%) and the lack of family (7.7%) types. Among nuclear families, those with relatives in near areas come first (62.2%), followed by the extended type (19.6%). There is a remarkably low rate of numerous families (9.3%), while binuclear families are exceptional. As familial demography is a factor to be considered in clinical practice owing to its influence in the familial function and resources, we propose the routine use of this classification in the family history.