Abstract In vivo tumor microenvironments are heterogeneous in their architecture, cellular contents, metabolic landscape, and extracellular matrix (ECM) composition. Moreover, these microenvironments can undergo temporal and spatial changes. This may alter tumor responses to anticancer therapies. To examine the complex and dynamic changes within tumors and in tumor microenvironments, we developed a computational single cell-based model of the three-dimensional multicellular organotypic cultures: Organoid3D. This model was calibrated to experimental data and reproduced growth dynamics and morphologies of four human breast cell lines: a nontumorigenic epithelial MCF-10A, a mildly tumorigenic MCF-10AT1, and metastatic MCF-10CA-1a and MCF-10CA-1d lines grown in various microenvironmental conditions. By explicitly including the fibril structure of the ECM in the model, we examined how ECM properties promote or suppress the growth of in silico tumor organoids. The quantitative integration of experimental data for organoids exposed to different concentrations of doxorubicin allowed us to derive hypotheses on relative importance of microenvironmental factors and chemotherapeutic treatments on organoid growth. Thus, the in silico organoid model interrogated with experimental data provides a tool for fast and broad hypotheses testing and presents an opportunity to explore experimental conditions beyond what is physically feasible in laboratory experiments. Citation Format: Katarzyna A. Rejniak, Sharan Poonja, Jessica Kingsley, Shreya Mathur, Ibrahim Chamseddine, Aleksandra Karolak, Dmitry A. Markov, Lisa J. McCawley. In silico organoids: A model for deconvolution of microenvironmental and drug effects on tumor growth [abstract]. In: Proceedings of the AACR Special Conference on the Evolving Landscape of Cancer Modeling; 2020 Mar 2-5; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2020;80(11 Suppl):Abstract nr A47.