Of the 35 patients with probable cases who seroconverted or showed a 4-fold rise in Chlamydophila psittaci MIF IgG titer, 33 (94%) also had 4-fold rises in CF antibodies with a Chlamydia genus–specific glycoprotein, 1 had no rise, and 1 had insufficient serum for testing. The gene discussed is MIF; the disease is cystic fibrosis.