Described here is a simple, rapid technique for isolating RNA from archived formalin-fixed, paraffin-embedded tissues (PETs). Using this modified acid guanidinium thiocyanate method, RNA sufficient as a template for the reverse transcription-polymerase chain reaction (RT-PCR) can be isolated in 2 hr from a single 20-microns-thick section of tissue. Spliced mRNA corresponding to portions of the estrogen receptor (ER) gene was successfully amplified from fixed, embedded human breast cancers containing increased amounts of ER protein. This method makes it feasible to safely and efficiently isolate RNA from large numbers of routinely processed paraffin blocks for retrospective studies of endogenous proteins such as hormone receptors, oncogenes, and viruses.