Known as Parkersburg Road, the state highway runs 2.34 miles (3.77 km) from MD 743 in Eckhart Mines north to MD 36 near Mount Savage in northwestern Allegany County.

The state highway crosses Porter Run on a narrow bridge before meeting U.S. Route 40 Alternate (National Pike).

MD 638 continues north as a two-lane undivided road and ascends the side of Federal Hill.

At Porter Cemetery Hill, the state highway turns northwest and descends into the valley of Jennings Run.

MD 638 passes through two sharp S-curves on either side of a 15-foot-wide (4.6 m) bridge over the Western Maryland Scenic Railroad.