We construct M-theory curves associated with brane configurations of SU(N), SO(N) and Sp(2N) 5D supersymmetric gauge theories compactified on a circle. From the curves we can account for all the existing different SU(N) field theories with Nf<=2N. This is the correct bound for N>=3. We remark on the exceptional case SU(2). The bounds obtained for SO(N) and Sp(2N) are Nf<=N-4 and Nf<=2N+4, respectively.