The Kleine Aa rises in East Westphalia 7 km north of the border to Hesse, between Bad Wunnenberg to the north-northwest and Marsberg in the east on the border between Brilon Plateau to the west and Sintfeld to the north. Its source is located 4.5 km west of Marsberg and about 5.4 km north-east of Brilon-Madfeld. Its water comes from a small wetland, located approximately 650 m northeast of the Totenkopf (502.6 m above sea level. NHN) on average 465 meters above sea level.