Large-scale ab initio multi-reference configuration interaction (MRD-CI) calculations are carried out to determine the vertical electronic spectrum of linear Cs. On the contrary to prior theoretical estimations based on the free electron model, the calculated energy for the (1)Sigma (+)(u) <-- X-1(+)(g) transition of 4.43 eV confirms recent measurements of 4.20 eV. The corresponding yet to be observed (1)Pi (u) <-- X-1(+)(g) transition is predicted to be located near 2.37 eV.