D & C Yellow No. 11 and Quinoline Yellow belong to a group of quinophtalone dyes with a common basic structure. D & C Yellow No. 11 is used mostly in plastics, spirit lacquers, coloured smokes and cosmetics, but it is also increasingly used as a dye in soaps and shampoos. Quinoline Yellow is used for dyeing wool, silk and nylon in cases where good light fastness is not required, but is most commonly used as a drug and food colouring agent. A patient, extremely sensitive to D & C Yellow No. 11 with a positive patch test reaction down to 0.00001%, also reacted to Quinoline Yellow at a test concentration of 0.1%. To establish if any cross-reactivity occurs between the two compounds, a high pressure liquid chromatograph was used for analysis, purification and separation of the two colours. The patient was then patch tested with the two colours and the pure fraction of Quinoline Yellow. The test results confirm a true sensitivity to Quinoline Yellow and to D & C Yellow No. 11, and may indicate cross-reactivity between the colours.