Between April 1993 and June 1994, 29 patients (pts) with unresectable, locally advanced, or metastatic non-small cell lung cancer were treated with a combination of p.o. trans-retinoic acid (TRA), 150 mg/m2/day, in three divided doses and s.c. IFN-alpha, 3 x 10(6) units/day. The age range was 41-80 years (median, 63 years). The Eastern Cooperative Oncology Group performance status was 0-1 (24 pts) and 2 (5 pts). Pts had advanced disease, refractory to conventional therapy (5 stage IIIB and 24 stage IV). Twenty-one pts had adenocarcinoma, six had squamous cell carcinoma, and two had large cell carcinoma. Only 3 pts completed 8 weeks of treatment, requiring neither interruption nor dose modification. Fatigue occurred in 88% of pts. A syndrome complex consisting of dry oral and nasal mucosa, recurrent sinus infections, and epistaxis occurred in 64% of pts. Grade II/III dermatitis was seen in 52%. Severe scrotal dermatitis was seen in 7 pts (47% of 15 males). Hypertriglyceridemia was moderate/severe in 11 pts, and 3 pts required gemfibrozil for levels up to 1660 mg/dl. Hematological toxicity was not encountered, and none of the pts had leukocytosis. One pt died with complications of myocardial infarction while on TRA/IFN-alpha. Twenty-five pts had more than 2 weeks of treatment and are evaluable for response; two pts died early with complications of cancer, and two pts declined to continue after only 3 and 5 days of treatment. Final assessment of response was by accepted clinical and radiological criteria at 8 weeks. There have been four objective responses: complete response, 2 (18+ and 17 months) and partial response, 2 (7 and 14 months). Responses were observed in all histologies. Combined differentiation treatment with TRA/IFN-alpha has modest but objective activity in non-small cell lung cancer. Toxicity is considerable. Additional studies to elucidate the biological basis of TRA/IFN-alpha and to define prognostic parameters predicting response are needed.