One hundred and sixty seven cases of pituitary adenoma were analysed using the immunocytochemical method of the Avidin-Biotin Complex (ABC), described by Hsu et al. (1981). Six pituitary anti-hormones were utilized: anti-prolactin (aPRL) at a 1:1,500 dilution; anti-growth hormone (aHGH) at a 1:4,000 dilution: anti-adrenocorticotrophic hormone (aACTH) at a 1:3,000 dilution; anti-thyrothrophic hormone (aTSH) at a 1:3,000 dilution; anti-luteinizing hormone (aLH) at a 1:1,000 dilution; and a anti-follicle-stimulating hormone (aFSH) at a 1:300 dilution. Incubation period was 14 to 16 hours at 4 degrees C. The survey of clinical, laboratory and radiological data of cases of pituitary adenomas was performed after reading the stained slides using the immunocytochemical method. Of the 167 cases of pituitary adenomas, 136 (81.4%) disclosed a positive immunoreaction to one or more anti-hormones, and the positivity index of neoplastic cells varied from 1 to 90%. The immunoreaction was positive exclusively to one anti-hormone in 80 cases (58.8%) and to two or more anti-hormones in 56 cases, and the association most frequently found was between both aPRL and aHGH. The positivity to the immunoreaction was distributed as follows: -100 cases were positive for aPRL, exclusively in 4 cases; -65 cases were positive for aHGH, exclusively in 22 cases; -31 cases were positive for aACTH, exclusively in 8 cases; -5 cases were positive for aTSH, exclusively in one case; -one patient presented an adenoma positive to aLH and another patient to aFSH.