The development of statistics on psychological diagnosis in asylum medical data included within public health reports was slow and far from linear. This chapter charts the history of statistics on psychological diagnosis in asylum medical data included within public health reports issued by the Commissioners in Lunacy and, from 1913, the Board of Control for Lunacy and Mental Deficiency. Medical statistics were published after the reforms to lunacy administration that took place in 1845, although these were somewhat crude and did not include statistics on psychological diagnosis. A gradual development occurred until the 1880s–1900s when nationally collated statistics on psychological diagnosis routinely appeared in annual Lunacy Commission reports. These were at times accompanied with detailed analysis that linked certain forms of insanity to context, gender and class of patient. This culminated in a set of reports issued during the first decade of the twentieth century that included the most complex breakdowns of national rates of diagnosis that had been presented. This progress finished with the introduction of the Board of Control for Lunacy and Mental Deficiency in 1913. The handing over of powers from the Commissioners in Lunacy to the Board of Control under the terms of the 1913 Mental Deficiency Act ended the routine presentation of statistics on psychological diagnosis in reports issued by lunacy administration. This chapter provides explanations for this abrupt halt which are found in developments in statistical theory at the beginning of the twentieth century and the statistical investigations into insanity by eugenicists. Ultimately, the interest in hereditary constitution came to overshadow concerns on diagnosis and classification within mental health statistics.