Background: The recommended dose for erlotinib of 150 mg was developed based on the maximum tolerated dose (MTD); meanwhile the suggested dose for gefitinib is only one third of its MTD. Studies suggest that the optimal biologic dose of erlotinib should be lower and dependent of the body mass index. Hereby we present results and toxicity with 75 mg/day dose in South American patients. Methods: We performed a retrospective review of patients with histologically proven (+) EGFR (+) mutation mNSCLC treated with 75mg/day erlotinib as starting dose at Centro Internacional de Estudios Clinicos. Clinical information, including toxicity grade 1-4, drug discontinuation, clinical evolution and radiological evaluation were revised. Overall survival from initiation of treatment was obtained from death certificate information. Results: Twenty-eight patients received 75mg/day of erlotinib as starting dose. Twenty-one (75%) patients were treated in first and seven (25%) in second line treatment. Mean age was 61years (range 36-89 years and nineteen (67%) patients were female. All patients had mutation positive EGFR, 22 (78 %) had Del19 and 6 (22%) exon 21 mutation. One patient had partial response and one had complete response. Progression free survival was 17 months and median overall survival 19 months. The main grade 1-2 toxicities were rash (33%) and diarrhea (25%). No grade 3-4 toxicity and no cases of drug discontinuation were reported. As of the writing of this abstract six patients are without progression and continue on treatment. Conclusions: In South American population with mutated mNSCLC, a dose of 75mg/day of erlotinib was well tolerated. This dose resulted in comparable benefits in PFS and OS when compared to those reported in the literature with the standard dose. Genetic polymorphism in metabolic enzymes and lower body mass index could explain the same efficacy at lower dose. More studies are needed to explore the use of adjusted doses of biological agents in different ethnic backgrounds.