The usefulness of amyloid-related serum protein (SAA) as an indicator of disease activity has been evaluated in 11 patients with rheumatoid arthritis (RA), 2 patients with psoriatic arthritis (PA) and 13 patients with ankylosing spondylitis (AS) prospectively studied during and after pregnancy. For comparison, SAA levels were recorded serially during and after pregnancy in 28 healthy pregnant women. SAA levels were unaltered by gestation and thus within the normal range during normal pregnancy, but were raised in healthy pregnant women with episodes of intercurrent infections. In RA and AS patients, SAA concentrations correlated to disease activity during and after pregnancy. Serial levels of SAA and C-reactive protein in healthy women and patients paralleled each other with the most pronounced inflammatory response displayed by SAA. We conclude that SAA is a sensitive and reliable indicator of inflammatory events both in the pregnant and non-pregnant state.