Follow-up information is presented on 37 women with SI and a history of severe acute salpingitis. It was found that 18 (49%) had clinical symptoms of intermittent or persistent back pain and stiffness indicating SI; 8 of them had serious and persistent discomfort. The rate of development varied greatly. The long-term prognosis appears to be good. Radiographic progression in the sacro-iliac joint was observed in 3 of the 11 patients who were re-examined. The findings indicate that salpingitis is a pathogenetic factor in SI in both HLA-B27 positive and negative women, but that the etiology and prognosis may differ between these two groups. The frequency of symptomatic SI tends to be higher among women carrying the HLA-B27 compared with those not having this antigen. In the latter group the course of disease was milder; the majority of the women had periodical symptoms or were asymptomatic.