The advances achieved during 1986 in the development and validation of the Spanish pressurized water reactor (PWR) core analysis system SEANAP-86 are presented and discussed. The advances were achieved through the cooperation of two research institutes (DENIM and JEN-CIEMAT) and two nuclear power plants (Almaraz and Ascó). The main features and advances of the core analysis system are summarized. The system is intended to provide the utilities with the in-house capability to analyze the core design, nuclear tests, and operation along reload cycles, as technical support for licensing and operation. The validation is done by an extensive comparison with the measurements available from the nuclear tests and in-core operation followup of four 900-MW(electric) PWR units (Almaraz-1 and -2, Ascó-1 and -2) along their first cycles and several reload cycles, as well as with the available design data. Good agreement is obtained for the broad sets of parameters and cycles analyzed, which qualifies the SEANAP-86 system as an effective core analysis support tool and encourages further developments, which are presently under way.