The genetic types of alpha1-antitrypsin (Pi types) were studied in 55 adults with classical or definite rheumatoid arthritis and 56 children with juvenile rheumatoid arthritis. A comparison was made with control groups of 211 healthy adults and 512 schoolchildren. Pi types MZ and SZ were significantly more prevalent in adults with rheumatoid arthritis than in the control adults. There was no difference between the patients with juvenile rheumatoid arthritis and the control children. Reduced concentrations of alpha1-antitrypsin, as associated with heterozygotes for the Z allele, may be a contributory factor to the development of rheumatoid arthritis or to the tissue destruction of rheumatoid arthritis.