It has been proposed that local production of estrogen may contribute to breast tumor growth, in part by regulating growth factor production. In view of this, we studied the expression of mRNAs for aromatase cytochrome P-450, the enzyme which catalyzes estrogen synthesis, and keratinocyte growth factor (KGF), a heparin-binding growth factor specific for epithelial cells, in breast tumors. In order to detect mRNAs of low-abundance, reverse transcription-polymerase chain reaction (RT-PCR) was used. RNA from breast tumors and normal breast tissue was reverse transcribed and then amplified using oligonucleotide primers for human aromatase or KGF. Twelve of 15 breast tumor samples yielded variable amounts of aromatase PCR product, but consistently strong KGF PCR signals. Of the three aromatase mRNA-negative samples, two gave weak KGF signals while one was negative for KGF. Both aromatase and KGF transcripts were also detected in all five normal breast tissue samples examined. These results indicate that a high proportion of breast tumors have the potential to produce aromatase and KGF, both of which could play important roles in their growth. The results also suggest that RT-PCR can be used to evaluate local expression of growth mediators in tumors.