Curative radiotherapy for cancer patients requires the use of radiation doses that are limited by the tolerance of the surrounding normal tissues. Unfortunately, these tolerance doses vary not only between tissues but also between individuals. In a small proportion of sensitive patients, exposure to radiation can lead to severe irreversible morbidity and even death several months to years after treatment. At present these radiosensitive patients can be identified only retrospectively. Here we describe a cytokine microarray technique that was used to identify differentially expressed gene transcripts in fibroblasts obtained from a small group of patients who suffered either negligible or severe normal damage to tissues after radiotherapy. If our preliminary findings can be confirmed, the availability of such markers may eventually allow the prediction of outcome prior to commencement of radiotherapy, and thus allow modification of radiotherapy protocols to minimize adverse late effects, without compromising tumor control. © 2002 by Radiation Research Society.