The fibrillar collagen network in tumor and normal tissues is different due to remodeling of the extracellular matrix during the malignant process. Collagen type I fibers have the crystalline and noncentrosymmetric properties required for generating the second-harmonic signal. The content and structure of collagen were studied by imaging the second-harmonic generation (SHG) signal in frozen sections from three tumor tissues, osteosarcoma, breast carcinoma, and melanoma, and were compared with corresponding normal tissues, bone/femur, breast, and dermis/skin. The collagen density was measured as the percentage of pixels containing SHG signal in tissue images, and material parameters such as the second-order nonlinear optical susceptibility given by the coefficient and an empirical anisotropy parameter were used to characterize the collagen structure. Generally, normal tissues had much more collagen than tumor tissues. In tumor tissues, a cap of collagen was seen at the periphery, and further into the tumors, the distribution of collagen was sparse and heterogeneous. The difference in structure was reflected in the two times higher coefficient and lower anisotropy values in normal tissues compared with tumor tissues. Together, the differences in the collagen content, distribution, and structure show that collagen signature is a promising diagnostic marker.