Although signals through the T cell receptor (TCR) are essential for the initiation of T helper cell activation, it is unclear what function such signals have during the prolonged T cell–antigen-presenting cell contact. Here we simultaneously tracked TCR-CD3 complex and phosphoinositide 3-kinase activity in single T cells using three-dimensional video microscopy. Despite rapid internalization of most of the TCR-CD3, TCR-dependant signaling was still evident up to 10 h after conjugate formation. Blocking this interaction caused dissolution of the synapse and proportional reductions in interleukin 2 production and cellular proliferation. Thus TCR signaling persists for hours, has a cumulative effect and is necessary for the maintenance of the immunological synapse.