Simultaneous exposure to retinoids and interferons can result in enhanced antiproliferative and differentiating effects on malignant lesions. We studied the toxicity and the potential efficacy of an association of high dose etretinate and Interferon-alpha (IFN-alpha) in squamous cell carcinomas of the lung, head and neck, the esophagus, cervix and the penis, as well as in transitional carcinomas of the bladder. The treatment consisted of etretinate (Tigason) 4 mg/kg/d on 2, 3, 4 and finally 5 consecutive days every other week and IFN-alpha (Roferon) 6 Mio IU sc. q.d. for 5 days every week. Of 24 patients enrolled, 23 were assessable for toxicity and 20 for response. With two occurrences of grade 3 cutaneous toxicity, the administration of etretinate (Tigason) 4 mg/kg/d on 5 consecutive days every other week and IFN-alpha (Roferon) 6 Mio IU sc. q.d. for 5 days every week was considered to be the MTD. Toxicity was mild otherwise, mostly at grades 1 and 2 level, causing fatigue, skin peeling and erythema, mucositis and cheilitis; 3 PR (partial response) and 8 SD (stable disease) were recorded. Of the responders, one patient had become resistant to cisplatin-based chemotherapy and the other two had at no time ever received systemic therapy. We conclude that the association of high doses of etretinate and IFN-alpha has moderate activity in squamous cell carcinomas, is well tolerated, and that IFN-alpha plays a role in the improved tolerance of the retinoid.