In the past, Ilmenau had been a small town of less than 2,000 inhabitants. Population growth began in the 19th century from 2,000 in 1800, 2,800 in 1850 and reaching 3,400 in 1870. Subsequently, the population grew rapidly to 10,400 in 1900 and 17,300 in 1940. After World War II, the population rose further to 20,000 in 1975 and to the peak in 1988 with a population of 29,500. The unfavourable economic situation in the old East Germany after reunification in 1990 resulted in a decline in the population, which fell to 27,000 in 2000 before stabilising.