Second harmonic generation microscopy was performed on both normal and diseased breast tissue. Differences in the collagen fibre shape between normal, benign and malignant breast tissue were compared and quantified using elliptical Fourier analysis. Principal shape analysis of these coefficients provided an understanding of the key differences in collagen fibre shape between the three tissue types. A Gaussian model was also used to associate the shape of the fibre with the probability that it had been sampled from malignant breast tissue. These results provide quantitative evidence for the alteration of collagen fibre shape in both benign and malignant breast tissue.