The population structures and demographic processes and their interrelations are considered as the main determinants of future developments of the population. This study evaluates the influence of mortality and fertility changes on the population structure by age and sex in Poland in the period of socio-economic transformation. The different impact for the urban and rural populations is also taken into account. In the 1990s, the radical changes of fertility and family pattern took place, similar to those observed in the developed countries since the 1960s and called 'the second demographic transition'. However, due to different economic setting, changes in Poland had their particular characteristics and dynamics. Between 1990 and 2000 number of people in Poland rose slightly (by about 464 thousands). In these years positive trends in mortality were observed, concerning adult and older people as well as infants, which resulted in the extended life expectancy. In the same time, number of births was constantly declining and fertility went down. Population ageing was progressing. Projections of the population in Poland with the cohort-component method revealed that without improvement in mortality the increase of population would be smaller by 60 thousands of people. The greater increase was possible due to decline in mortality of men but the projected number of women in 2000 was also greater than observed. The residents of cities benefited more from the decline in mortality than the rural population. The most influenced by the mortality decline were children aged 0-10 years, men in age groups 45-55 and older people of both sexes (60-84). No increase was observed in the oldest age group (85+). Distributions of gaining and losing age groups were different for urban and rural areas. Improvements in mortality hardly influenced population ageing. With the constant mortality at the level of 1990, ageing at the bottom of the age pyramid would be faster than it was observed. Greater projected number of births than observed was the straightforward result of the fertility decline. The differences in the observed and projected numbers of births went up with time. The fertility decrease in the period under study contributed to population ageing, especially at the bottom of the age pyramid. Indirectly, it also influenced the structure at the top. Since the analysis was based on period measures, one can expect that the cohort fertility will be higher than the level shown by period rates. There are indications that a decline in fertility are mostly the result of postponing births rather than an established pattern of childlessness.