Several styling refinements hit DeVille (and Fleetwood) for 1978. A new, bolder grille pattern in three-row checkerboard style was repeated to the left and right of the license plate below. The new pattern had fewer (and more square) holes than before. Cadillac's script was above the grille's header bar (mounted on the body), on the driver's side. A new solid-color wreath-and-crest ornament rode the hood. New, extra-slim vertical taillamps in tall chrome bumper tips had thin backup lamps inset in the middle and built-in side marker lamps. Back bumpers were new. DeVille's padded vinyl roof had a custom-trimmed backlight (rear window). Rust-prevention measures included Zincrometal panels and bimetal (stainless steel on aluminum) moldings, plus microencapsulated epoxy-coated screws in key areas.

Underhood was the 425 cu. in. (7.0-liter) V-8 introduced the year before. Six-way power driver's seat controls were now in the door armrest. New interiors included seatback pockets. Standard on all models: an AM/FM Signal-Seeking stereo radio with scanner and disappearing antenna. Coupe and Sedan DeVilles also had automatic climate control, power windows, and much more. Sunroof, wire wheel discs and 50/50 Dual Comfort seats were available. Nine interior color selections were offered in new Random velour or Hampton woven cloth. Up to a dozen Sierra Grain leather choices came with color-coordinated carpeting. Options included

• a custom-trimmed padded vinyl top
• rear deck accent stripes
• color-keyed bodyside moldings
• chromed wire wheels
• electronic leveling
• chrome accent moldings.

Coupe DeVille could have the Cabriolet roof package, including a chrome accent strip to highlight the Elk Grain vinyl top, Cadillac crests, and a French seam around the back windows -- offered in 16 coordinated colors. A DeVille d'Elegance special edition featured pillow-style 50/50 Dual Comfort seats in new Random velour (antique dark green, light beige, light blue. or dark mulberry). Upper doors and front seatback assist straps wore matching Random velour. High-pile carpeting extended part way up the doors. The glove box door held a "DeVille d'Elegance" insignia, as did the pillar behind back side windows. The hood, door, and decklid were striped. Side moldings held vinyl inserts that matched the body color. Other extras:

• opera lamps
• three roof-mounted assist straps
• door pull handles

The d'Elegance trim package came in 21 body colors. Arriving later were Phaeton packages that featured a simulated convertible top (down to authentic-looking welts and stitching). Offered in Cotillion White, Platinum or Arizona Beige (with contrasting top colors), these were identified by "Phaeton" nameplates on back fenders and wire wheels.