A method is presented for obtaining assumption-free estimates of the number of nuclei in the trophoblast of the human placenta and for defining the size of the trophoblast proliferative unit (TPU). The method relies on the disector, a stereological device for counting arbitrary particles in 3-dimensional space using pairs of parallel sections separated by a known distance. It is applied to investigate factors which contribute to trophoblast growth from 13 weeks of gestation to term. Physical disectors were sampled systematically using adjacent 4-4.6 microns thick paraffin sections. Nuclei in the trophoblast (syncytial and cellular) were counted if they appeared in an unbiased counting frame on one section but were absent from the adjacent section. Nuclear packing densities were converted to absolute numbers of nuclei by using placental volume as the reference space. At 37-39 weeks, the average placenta contained 6.4 x 10(10) trophoblast nuclei of which 90 per cent were located within the syncytium and the remainder in the cytotrophoblast. From a knowledge of total trophoblast volume, it was found that each nucleus is associated with 970 microns3 of trophoblast and each cytotrophoblast cell with 11,000 microns3. The latter may be regarded as the volume of a TPU. From 13 weeks of gestation to term, there was a ninefold increase in nuclear number but the trophoblast volumes associated with nuclei, including the size of the TPU, remained constant. Growth of trophoblast is purely hyperplastic and occurs by recruitment of new TPUs.