Leonardo da Vinci designed several versions on the theme of St. Anne in Three, supposedly between 1499 and early 1510s. Of them two actual size works are pre?served. The one is a cartoon in the National Gallery, London, which shows the Vir?gin sitting on the right lap of St. Anne and the Christ child playing with St. John the Baprist. The other is a painting in the Louvre, Paris, in which compositional relationship of the figures underwent great change. While me figures of the Virgin and St. Anne in the London cartoon namely are arranged next to each other on the same level, those in the Louvre painting form a slanting movement line together with the Child Child and a lamb. This is achieved particularly by interlocking and reacting postures and gestures of the figures. This difference in artistic approach has been explained only in terms of Leonardo`s concern with compositional problems, with the conclusion that the Louvre painting was the final and the most intricate solution ever. This study however, attempts to show that Leonardo designed various compositions not just for stylistic reasons but most of all owing to his iconographical interests. Investigations on the icono-graphical tradition of the theme of St. Anne in Three and on the cult of the saint around 1500 resulted in the following observations. The London cartoon and the Louvre painting of Leonardo both deliver the notion of Immaculate Conception by showing the maternal family line of Jesus, which is ultimately connected with the concept of Incarnation. The London cartoon, repre-senting the Christ Child with the gesture of Benediction to St. John the Baptist and the index finger of St. Anne pointing w the sky, seems in particular w lay a stress on the divinity of Jesus Chirst. Its horizontal structure of figures then appears to belong to a type of St. Anne in Three that was invented Just before 1500 in the north of the Alps. h would reflect the equality of Madonna and St. Anne and thus the high position that St. Anne gained through her popularity. Hence we could say that the London Cartoon adopted a contemporary version of the theme, a popular type of the period. The Louvre paining on the other hand adopted the posture of Maria sitting sideways deeply in the lap of her mother and thus kept the vertical relationship of the figures, This has its origin in an ancient iconographical type of Sr. Anne in Three, Hodegetria, and we could safely assume that the Louvre paining remained faithful to the original theological idea of St. Anne in Three, expressing the theory of the Incarnation on the basis of Immaculate Conception, which could only be explained through the maternal family line of Jesus Christ.