Frequencies of alpha-1-antitrypsin (Pi) phenotypes were studied in 100 female and 100 male Swedish patients with classical rheumatoid arthritis and compared with the population frequencies. A significant increase of rare Pi types (MS, MZ, MF and SZ) was found among the patients. However, the increase concerned mainly Z heterozygotes and was more strongly pronounced in male patients. The M-subtypes showed no association with rheumatoid arthritis. Previous investigations of Pi types in rheumatoid arthritis have shown somewhat variable results. The results so far indicate, however, that an association between the Z allele and rheumatoid arthritis is likely to exist, while the evidence for a relationship between rheumatoid arthritis and other Pi alleles is considerably weaker.