WIS 138 starts in Cooksville, an unincorporated community with an Evansville address.

It runs north into Dane County towards Stoughton, although it makes one S-bend on the way.

It runs concurrently with US 51 to the west side of the city, then continues west in a straight line towards Oregon.

[5][6] In late-1978, US 14 was moved onto a newly built expressway around Oregon.

As a result, WIS 138 no longer needed to travel southwestward to get to US 14.