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The measurement of the effect of cisplatin on DNA has become feasible with the development of antibodies against DNA adducts. In a phase II dose escalation trial with concomitant radiotherapy and daily cisplatin in lung cancer, we found that patients with high DNA adduct levels measured in the buccal mucosa had a much higher survival rate than patients with a low or undetectable amount of cisplatin-DNA adducts. The use of this assay may therefore allow the selection of individual patients for concomitant treatment with cisplatin and radiotherapy, as has been shown to be effective in randomized trials in patients with lung, head and neck, and cervix malignancies. To predict the response to radiation treatment, assays have been developed for tumor growth potential by measuring the labeling index after intravenous injection of IdUrd or by estimating cyclin D1 expression. Intrinsic radiation sensitivity of human tumors can be estimated by conventional techniques, which are probably too slow or cumbersome for routine use, or with more rapid assays, such as those for chromosome damage with fluorescent probes. These assays should be able to guide us in the adaptation of the individual radiation doses that should be applied and to select patients for an accelerated or hyperfractionated regimen. Pretreatment levels of apoptosis may also be helpful in predicting treatment outcome, although the data so far show inconsistent results. A better understanding of the signal transduction pathways involved in radiation-induced apoptosis may help in the design of studies aimed at modulating the apoptotic response, thereby increasing cell kill. We have recently shown that alkyllysophospholipids, which inhibit mitogenic signaling, induce apoptosis in a variety of tumor cell lines. In combination with ionizing radiation, these compounds cause an enhancement of apoptotic cell kill. This type of a signaling-based intervention could form the basis for new therapeutic strategies. The role of hormonal therapy in breast cancer patients, both in an adjuvant setting and for the treatment of disseminated disease, is becoming increasingly important. The development of a functional assay for the estrogen receptor (ER-FASAY), based on a yeast growth assay, provides a better way than the classical immunohistochemistry assay of estimating abnormal function of the receptor in tumors. These assays are simply examples, illustrating how clinicians could improve the therapeutic outcome for their patients by implementing knowledge obtained in the laboratory in clinical decision making. With further optimization of these assays, this holds the promise for the future that the treatment for each patient can be tailored rationally to the biology of the individual.