Inter Milan brushed off Spezia to make it two wins from two at the start of the Serie A season.
Romelu Lukaku, in his first game at San Siro since re-joining on loan from Chelsea, headed the ball back to Lautaro Martinez to rifle in a fantastic opener from outside the box.
Lukaku then hit the crossbar with a header late in the first half.
Hakan Calhanoglu blasted in a first-time effort and Joaquin Correa added a third from Edin Dzeko's pass.
AC Milan, who beat Inter to last season's title, visit Atalanta on Sunday.