While the visitors have been on a poor run, they remain second in the table and cannot have anticipated being turned over 2-0 by a side who have taken a solitary point from the previous 15 available to them.
But Chievo have a good record in the fixture and sealed victory in the first half thanks to goals from Boukary Drame and Cyril Thereau, while Edinson Cavani missed a penalty for Napoli.
Napoli produced a reasonable chance in the seventh minute, Christian Maggio getting his head to Lorenzo Insigne's cross but just failing to get his attempt on target.
Soon after Marek Hamsik combined with Juan Zuniga, but the latter was wayward with his shot.
In the 12th minute, the hosts converted their first chance to take the lead.
It arrived in the aftermath of a corner, with Drame striking from distance to beat Morgan De Sanctis at his bottom corner.
A second almost followed immediately, but Alberto Paloschi's header came back off the crossbar.
Napoli gradually got back into it, Cavani producing a fine run but a poor shot and Insigne skimming the post with a 25-yard effort.
Cavani tested Christian Puggioni's reflexes with a lofted effort in the 38th minute, but Chievo again upset the run of play two minutes from the break.
Nicolas Frey provided the pass for Thereau and he fired home despite De Sanctis making decent contact.
It took Napoli until the 58th minute to fashion their best response, Dario Dainelli giving away a penalty only to see Puggioni spare him by saving from Cavani.
That took the wind from Napoli's sails and the only realistic prospects of a third goal came from the home side.
Drame and Paloschi had the best chances as they sought to add to their earlier efforts, but neither man was able to convert.