Early results in 1988 showed the leftist opposition candidate Cuauhtemoc Cardenas well ahead of President Salinas de Gortari of the ruling Institutional Revolutionary Party (PRI). The PRI-controlled Federal Electoral Commission then announced its computers had 'gone down'. By the time the system was 'restored,' Mr Salinas was ahead and won with a fraction over 50 per cent of the vote, the narrowest win in his party's history.
Unisys, the US-based multinational computer corporation, which provided the vote- tallying system, is to investigate what happened.
Mr Cardenas, who never recognised the result or Mr Salinas's presidency, is running again against Ernesto Zedillo of the PRI and the conservative candidate Diego Fernandez de Cevallos.
Unisys will not be involved this year. IBM won the contract for a system including a register of the 45 million voters and a triple back-up.