The effect of altered inflammatory activity, as reflected by acute phase protein levels, on the concentration of lipoprotein(a) was analysed in 51 patients with rheumatoid arthritis (RA), and in 25 patients with polymyalgia rheumatica (PMR). Despite significantly decreased acute phase proteins in RA, the Lp(a) concentration increased significantly between the two observations in the group as a whole. In one subgroup with initially high Lp(a) levels, large intraindividual variations between the samplings and a high correlation to the ESR changes were found. Another subgroup with initially low levels showed small variations of Lp(a) without correlation to any acute phase reactant. In the PMR group, Lp(a) decreased in line with diminishing acute phase reactivity.