We consider Brownian motion in a circular disk Ω, whose boundary ∂Ω is reflecting, except for a small arc, ∂Ω a , which is absorbing. As ε = |∂Ω a |/|∂Ω| decreases to zero the mean time to absorption in ∂Ω a , denoted Eτ , becomes infinite. The narrow escape problem is to find an asymptotic expansion of Eτ for ε ≪ 1. We find the first two terms in the expansion and an estimate of the error. The results are extended in a straightforward manner to planar domains and two-dimensional Riemannian manifolds that can be mapped conformally onto the disk. Our results improve the previously derived expansion for a general smooth domain, Eτ = |Ω| Dπ log 1 ε + O(1) , (D is the diffusion coefficient) in the case of a circular disk. We find that the mean first passage time from the center of the disk is E[τ | x(0) = 0] = R 2 D log 1 ε + log 2 + 1 4 + O(ε) . The second term in the expansion is needed in real life applications, such as trafficking of receptors on neuronal spines, because log 1 ε is not necessarily large, even when ε is small. We also find the singular behavior of the probability flux profile into ∂Ω a at the endpoints of ∂Ω a , and find the value of the flux near the center of the window.