If a cavity does not hurt, can I leave it untreated?

It needs treatment even if it does not hurt. A cavity causes little pain until it approaches the nerve, and by the time it hurts it has often already progressed close to the nerve (pulp). By stage, 1) an early cavity confined to the enamel is finished with a simple filling, 2) decay reaching the dentin needs a restoration such as resin or an inlay, and 3) decay reaching the nerve requires root canal treatment and then a crown, which greatly increases both treatment time and cost. The earlier it is found at a check-up, the simpler the treatment.