Possibly Cullen's best known wine, this flagship red is named in honour of Diana Madeline Cullen, who along with husband Dr Kevin John Cullen, planted their first Cabernet Sauvignon vines in 1971, in Wilyabrup, a sub-region just north of Margaret River township.

Additional varietals were planted in 1971, 1976 and finally in 1988, to both red and white varietals, including Merlot, Cabernet Franc, Chardonnay, Sauvignon Blanc and Petit Verdot, and a final planting in 1988. The Mangan Vineyard, situated just across the road from the Cullen Vineyard, was planted between 1995 and 1997, and contributes Malbec amongst other red and white varietals.

The wines these days are made by their daughter Vanya Cullen, who took over the role of winemaker in 1989, and has consistently been at the forefront of advancing Australian wine knowledge, just as her parents once did. Cullen received organic certification in 1998, and began the conversion to full biodynamic viticulture in 2003, with certification received in 2004 for the Cullen Vineyard, and 2008 for the Mangan Vineyard.

The Cullen Cabernet Merlot was renamed 'Diana Madeline Cabernet Merlot' in 2003, in honour of her passing, and her enormous contribution to the Australian Wine Industry, being chief winemaker at Cullen from 1981 until passing the reigns to Vanya in 1989, and along the way becoming the first woman winemaker to win a trophy at the Royal Perth Show, in 1982.

The 'Cabernet Merlot' aspect was removed from the label in 2005, and the wine has simply been known as 'Diana Madeline' ever since.

Cullen's focus on viticulture, first and foremost, lends an elegance and intense concentration to the wines, with a focus on achieving tannin ripeness and balanced fruit. The Diana Madeline has excellent cellaring potential, and is indisputably a world-class Cabernet blend, and certainly one of the great wines of Australia, with significant interest on the secondary market for older vintages.