Twin studies attract both clinicians and geneticists because of the value of the twin method in helping unravel the genetic predisposition to diseases and the role of environment in their causation. In the field of rheumatology, there are many case reports on twins concordant or discordant for diseases. Interesting as such reports may be, very few generalizations can be made from them. The concordance rate among monozygotic (MZ) twins indicates the maximum level of genetic contribution. Based on studies of systematically compiled twin series, the concordance rate is about 15% for rheumatoid arthritis; the rate is probably of the same order of magnitude for systemic lupus erythematosus (SLE). The fine specificity of antinuclear antibodies of MZ twins at least one of whom is affected by SLE is very similar, despite somatic generation of immune diversity. Up to now, twin studies have provided little information on the role of environmental factors in rheumatic diseases. A notable exception is a case-control study of MZ twins discordant for smoking that reinforced the role of smoking as a cause of lumbar intervertebral disc degeneration.