The main result of this paper is the proof for elliptic modular threefolds of some conjectures formulated by the second-named author and shown by Jannsen to be equivalent to a conjecture of Beilinson on the filtration on the Chow groups of smooth projective varieties. These conjectures are known to be true for surfaces in general, but for elliptic modular surfaces we obtain more precise results which are then used in the proof of the conjectures for elliptic modular threefolds.Let φ : E → M be the universal elliptic curve with level-N structure, whose smooth completion is an elliptic modular surface E. An elliptic modular threefold is a desingularization 2 E of the fibre product E × M E. The first main result is that there exists a decomposition of the diagonal ∆( 2 E) modulo rational equivalence as a sum of mutually orthogonal idempotent correspondences π i which lift the Künneth components of the diagonal modulo homological equivalence. These correspondences act on the Chow groups of 2 E, and secondly we show that π i · CH j ( 2 E) = 0 for i < j or i > 2j; the implication of this is that there is a filtration on CH j ( 2 E) that has j steps, as predicted by the general conjectures. The third main result is that the first step of this filtration, the kernel of π 2j acting on CH j ( 2 E), coincides with the kernel of the cycle class map from CH j ( 2 E) into the cohomology H 2j ( 2 E); which is to say that there is a natural, geometric description for this step of the filtration. We also identify F 2 CH 3 ( 2 E) as the Albanese kernel. As a by-product of our methods we also obtain some information about the Chow groups of the Chow motives for modular forms k W defined by Scholl, for k = 1 and 2, for example that CH 2 ( 1 W) = CH 2 Alb (E), and that CH 3 ( 2 W) = F 3 CH 3 ( 2 E) lives at the deepest level of the filtration, within the Albanese kernel.Typeset by A M S-T E X 1 1.1.6. The functor V(S) → M(S). There is a natural contravariant functor from V(S) to M(S), given by associating to a morphism f : X → Y of smooth projective S -schemes the class of the transpose of its graph, [ t Γ f ] ∈ CH d S (Y ) (Y × S X, Q), and associating to X in V(S) the object (X, [∆(X)]), where ∆(X) denotes the diagonal in X × S X . When S = Spec k we write h(X) := (X, [∆(X)]).