Cell kinetic information is an important adjuvant to histological typing and grading of human brain tumours. In the current study detection of the proliferating potential of adenomas was attempted using silver colloid technique for argyrophilic protein associated with nucleolar organizer regions (AgNORs). The expression of nucleolar organizer regions was assessed on paraffin sections in 28 pituitary macroadenomas which included all the main types. Nucleolar organiser regions have been quantified with computer-aided image analysis system in order to evaluate the mean area of AgNORs and their number relative to an internal reference, the number and areas of clusters of AgNORs and the area of the nucleus. In the pituitary macroadenomas there were 1.48 +/- 0.30 (mean +/- SEM) AgNORs per cell nucleus and corrected values were 1.66 +/- 0.27 (mean +/- SEM) AgNORs. The mean nucleus area was 867.18 +/- 203.91 pixels. The mean AgNORs size was 54.50 +/- 34.80 pixels. Correlation of nucleus area/AgNORs area was: r = 0.7085. On the number of the AgNORs it was possible to distinguish 3 subgroups of macroadenoma with different proliferative potential: low < = 1.367 (1.479); medium 1.367-1.733 (1.479-1.864) and high > 1.733 (1.864) AgNORs per cell nucleus and corrected number of AgNORs per cell nucleus respectively. Thus, this method provides useful information about the proliferative potential of human pituitary adenoma.