In a dose escalation study, CIS-diamminedichloroplatinum II (cisplatin) was combined with a standard dose of external beam irradiation in 15 patients with localized non-small cell lung cancer (NSCLC) and 16 patients with fixed or recurrent localized adenocarcinoma of the rectum. Cisplatin was given 5 days a week during irradiation using an outpatient portable infusion pump system, at doses of 3.2 mg/m2/24 hr in 15 patients, 4.0 mg/m2/24 hr in 13 patients, and 5.0 mg/m2 24 hr in 3 patients. Twelve of 15 patients with NSCLC received 66 Gy in 33 fractions in 6 1/2 weeks; one received 46 Gy followed by a surgical resection; for the other two patients treatment was discontinued after 50 Gy and 64 Gy, respectively, because they developed distant metastases. The 16 patients with rectal carcinoma received a preoperative dose to the pelvis of 45 Gy in 25 fractions in 5 weeks. Of 12 patients who underwent laparotomy, 10 had a surgical resection, 2 with close or positive surgical margins. Four patients who had resections received an intraoperative electron boost. Of the two patients who did not undergo resection at laparotomy, one received an intraoperative electron boost, the other a boost with interstitial iridium-192. Among the four patients with rectal adenocarcinoma who were not candidates for surgery because of advanced local disease, two had further external beam therapy up to 59.4 Gy, and two had no further therapy. Major toxicity was site-specific, with esophagitis predominating in the patients with NSCLC, diarrhea in the patients with rectal carcinoma, and nausea experienced by both. Cisplatin dose and toxicity seemed to be related. The maximum tolerated dose for low-dose continuous infusion cisplatin given 5 days/week in these patients was 3.2 mg/m2/24 hr combined with 66 Gy in patients with NSCLC and 4.0 mg/m2/24 hr combined with 45 Gy in patients with rectal carcinoma.