Aluminium (AlS) and zinc (ZnS) concentrations in serum were measured in patients with rheumatoid arthritis (RA) with and without Al-containing drugs and compared with levels in healthy controls. In RA patients without Al-containing antacids, the AlS levels did not differ from those of the healthy controls (mean 9.9, SEM 0.9 microgram/l and mean 8.6, SEM 0.5 microgram/l). However, the AlS levels in RA patients treated with Al-containing drugs were significantly (p less than 0.05) higher than in the other two groups (mean 14.1, SEM 1.1 microgram/l). Serum zinc (ZnS) levels in both RA groups were significantly (0.01 less than p less than 0.025) lower than in the healthy controls (mean 10.5, SEM 0.4 mumol/l and mean 10.2, SEM 0.4 mumol/l for the RA groups with and without Al-containing drugs, and in healthy volunteers mean 14.4, SEM 0.6 mumol/l). The difference in AlS levels might be due not only to the intestinal absorption of Al from Al-containing drugs, but also to a slightly impaired renal function, serum creatinine being slightly but significantly higher (p less than 0.05) in the Al-treated RA group than in non-Al-treated one. This difference in serum creatinine between the two RA groups might be explained by the more frequent use of cimetidine in the Al-treated RA group.