The usefulness of the in vitro chemosensitivity test, the collagen gel droplet embedded culture drug- sensitivity test (CD-DST, Int J Oncol 11: 449, 1997), in cisplatin-based combined chemotherapy for postoperative recurrent tumors in non-small cell lung cancer (NSCLC) patients was retrospectively analyzed. CD-DST data for cisplatin (or carboplatin), etoposide, 5-fluorouracil, mitomycin C, and vindesine were obtained in 311 surgically resected primary lesions. Of them, 25 patients were practically treated with first-line cisplatin- or carboplatin-based chemotherapy for postoperative initial recurrence. Nine (36%) of them responded to the combined chemotherapy for recurrent lesions, including one with complete remission, whereas 16 did not, with no change in 5 and progression in 11. Seven (70%) of 10 patients receiving combined chemotherapy using two or three in vitro sensitive chemoagents showed good responses, whereas there was no responder among the patients receiving chemotherapy including no in vitro sensitive chemoagents. In particular, of 11 patients showing good sensitivity to cisplatin or carboplatin on CD-DST, 8 (73%) responded to chemotherapy, whereas only one (7%) of 14 patients showing cisplatin- or carboplatin-resistance on CD-DST was a responder. Thus, CD-DST results for the chemoagents, especially cisplatin or carboplatin, correlated with chemotherapeutic response, indicating that the CD-DST analysis of surgically resected primary NSCLC tumors is a practically useful indicator of the clinical effect of first-line cisplatin-based combined chemotherapy for postoperative recurrence.