Abstract Introduction: Squamous skin cancer is one of the most common neoplasia worldwide and may be preceded by Bowen's disease (BD) and Actinic Keratosis (AK). These preneoplastic lesions can be treated with photodynamic therapy (PDT). PDT causes cell death mainly by reactive oxygen species (ROS) produced through a combination of O2, a photosensitizer and light. BD and AK have well overcomes after PDT. However, high recurrence rates of some lesions have been reported. Objective: To generate resistant cells to PDT as an in vitro model for future studies. Methodology: HSC-1 cells from human squamous cell cancer (SCC) of skin were used. Cells were treated with a total of 16 cycles of PDT. Every PDT cycle was carried out incubating cells with methyl aminolevulinate (MAL), as a protoporphyrin IX (PpIX) precursor, for 4 hours in the dark. Then, cells were irradiated with red light (630 nm). The light fluence used for the four first cycles was 1 J/cm2, for subsequent cycles light fluence increased to 2, 3 and 4 J/cm2. Cell viability was evaluated 1 and 24 hours after PDT by flow cytometry and MTT assay respectively. Intracellular PpIX was detected by flow cytometry and the PpIX content was quantified using a standard curve. By flow cytometry using a CM-H2DCFDA probe intracellular ROS was detected. Results: After 24 hours of PDT (4 J/cm2), viability in parental HSC-1 cells was 5%, meanwhile in resistant cells was 70% (P < 0.05). On the other hand, 88% of parental cells produced PpIX (40 ng/mg protein) and 18% of resistant cells produced PpIX (17 ng/mg protein) (P < 0.05). No significant differences were found in ROS detection in both parental and resistant cells. Conclusion: The results indicate that it was possible to obtain HSC-1 cells two-fold resistant to PDT compared to their parental cells. Acknowledgments: Financial support by CONICYT. CONICYT-PCHA/Doctorado Nacional/2012-21120665, CORFO 09CN14-5960, Corfo N°12IDL2-18157, FONDECYT N° 11150802, FONDECYT N°11150622. Citation Format: Daniela Leon, Ramon Silva, Natalia Inada, Cristina Kurachi, Carmen Gloria Ili, Priscilla Brebi, Juan Carlos Roa. Resistance to Photodynamic Therapy in Non-Melanoma Skin Cancer Cells. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr B30.