Hayek had high regard for Marx in technical economic theory and considered him a predecessor in his business cycle theory. [...] It was not in technical economic theory that the classical Austrians disagreed with Marx. So towering a figure in history is Marx that discussion of his thought in summary form is always difficult, for there is so much that he said and that others have said about him. At the same time, so tendentious, ill-spirited, and just plain wrong a thinker was Marx that it is surprising that he may have had some of the influence attributed to him. Hayek's opposition to Marx was in the realm of practical political emanations from Marx's thought. Here he considered Marx's influence to have been wholly pernicious.

Hayek's Journey: The Mind of Friedrich Hayek (2003) - Ch. 12 : Marx, Mill and Freud