This paper examines the probability of interactions occurring between drug lesions and radiation lesions in DNA for the cytotoxic and radiosensitizing agent cisplatin. The number of cisplatin-induced DNA adducts and radiation-induced strand breaks after a given dose of each agent are known for given cell systems, from which the probability that these lesions will interact can be estimated. Results of these calculations indicate that the probability of interaction could be high, depending on the distance over which two lesions can interact and the probability of repair of the interaction lesion. Calculated lesion numbers have been compared with known data on radiation modification, including illustrations of inconsistencies. In the second part of the paper, ways in which combined therapy with cisplatin and radiation can be improved are described. Development of methods to predict which types of tumor and which individual tumors within a given type are sensitive to the cytotoxic and radiosensitizing effects of the drug would aid rational selection of patients for combination treatments. Immunocytochemical methods sensitive enough to monitor cisplatin-DNA interactions in patients are available and may be useful in this context. The delivery and maintenance of higher tumour concentrations of radiosensitizer offers a further possibility for improvement. Studies of intratumoral injection of cisplatin have shown promise for achieving this goal while limiting normal tissue toxicity.