Summary Specimens from 45 patients with previously-untreated non-small cell lung cancer (NSCLC) were tested for in vitro chemosensitivity to ten drugs utilising the DiSC assay, which measures cell kill in the total (largely non-dividing) tumour cell population. Thirty-five assays were successful and 25 patients with advanced disease subsequently received chemotherapy with the 'best' three drugs selected by the assay. Six patients were Karnofsky performance status 60 or less and the median pretreatment weight loss was 8.5%. Nine patients had a partial response (response rate = 36%; 95% confidence interval = 17-55%) and the median survival of all patients was 202 days. Specimens from responding patients were significantly more sensitive in the assay to drugs in general (especially to etoposide and to 'natural product' drugs) and to the drugs used in treatment than were specimens from non-responding patients. In vitro drug resistance differences between responding and non-responding patients were of greater significance than were differences between other clinical and laboratory measurements. Assay results classified patients into two cohorts, having relatively high and low probabilities of responding to chemotherapy. Assay results also identified patient cohorts with above average and below average durations of survival. Five patients (20%) were found to have tumours with extreme drug resistance (EDR), defined as assay results for the average of all ten tested drugs falling greater than one standard deviation more resistant than the median for all tumours assayed, and none of these patients with EDR responded to chemotherapy.No single program of chemotherapy has emerged as the standard of treatment for patients with advanced non-small cell lung cancer (NSCLC). Randomised comparisons have covered a ranged of options including multidrug and single drug therapy and no chemotherapy (Mulshine et al., 1986;Hansen, 1987). Therapeutic choice has often been without significant impact on survival. Where statistically significant survival improvement has been reported the magnitude of the benefit has been modest and caution seems appropriate in weighing this benefit vs the toxicity from the treatment.One hope for improving treatment outcome has been the provision of customised treatment on the basis of individual tumour properties. From December 1983 through August 1986 we investigated this approach with a pilot study of individualised chemotherapy for NSCLC selected on the basis of an in vitro drug resistance assay. The assay chosen was a dye exclusion assay (the DiSc Assay) which has received extensive study in haematologic neoplasms (Weisenthal et al., 1984;Weisenthal et al., 1986;Bird et al., 1985;Bosanquet et al., 1983;Bird et al., 1986;Tidefelt et al., 1989;Kirkpatrick et al., 1990;Lathan et al., 1990;Bosanquet, 1991), but more preliminary evaluation in solid tumours Gazdar et al., 1990).We are now reporting the final results of this trial.