By regulating cyclin-cyclin-dependent kinase (CDK) complex activity, individual CDK inhibitors (CDKIs) are potential tumor suppressors. One of the CDKIs, p27/Kip1, binds to a variety of CDK-cyclin complexes. A link between loss of p27/Kip1 function and development of pituitary tumors was suggested by the formation of pituitary tumors in almost all mice with germline deletion of the p27/Kip1 gene. However, genetic aberrations in the p27/Kip1 locus have not been analyzed in human pituitary tumors. We investigated eighteen non-functioning and GH-secreting pituitary tumor samples for p27/Kip1 mutations by single-strand conformational polymorphism (SSCP) following PCR. We found five abnormally migrating samples on the PCR-SSCP analysis. The sequence of these samples revealed a polymorphism of codon 109 (Val-->Gly), which has been previously described. No other structural changes of p27/Kip1 were found in these pituitary tumors within the coding region. In addition, no difference in p27/Kip1 protein levels in pituitary tumor tissues compared with normal pituitary tissues was demonstrated by immunostaining. These data suggest that both p27/Kip1 mutations and decreases in p27/Kip1 protein levels are infrequent in the development of pituitary tumors.