  The low-temperature specific heat of La_{2-x}Sr_xCuO_4 (LSCO) single crystals
in magnetic field H up to 12 T has been examined over a wide range of doping
(0.063=< p =<0.238). From this we have mapped the upper critical field H_{c2}
of LSCO across the entire superconducting diagram. It is found that the H_{c2}
shows a doping dependence similar to that of the critical temperature T_c. We
have discussed the implications of the result and proposed that there may be an
effective superconducting energy scale responsible for the H_{c2} behavior in
the underdoped region.
