The cause of ankylosing spondylitis (AS) was investigated through 11 Karl Popper sequences. “Popper sequences” provide a powerful method of investigating a scientific problem. A “Popper sequence” consists of a “problem”, “tentative theory”, “error elimination” which then leads to a “new fact”. The 11 “Popper sequences” establish that: (1) HLA-B27 lymphocytes injected into a rabbit evoke antibodies against Klebsiella, (2) anti-HLA-B27 tissue typing sera bind to Klebsiella antigens, (3) total serum IgA is elevated in AS patients, (4) antibodies to Klebsiella are present in AS patients from 16 different countries, (5) antibodies to Klebsiella in AS patients are disease specific, (6) Klebsiella bacteria can be grown from fecal cultures, (7) the sequence QTDRED found in HLA-B27 resembles a sequence DRDE found in pullulanase-D enzyme of Klebsiella, (8) Klebsiella pullulanase-A contains a sequence which resembles type I, II, and IV collagens, (9) sera from AS patients have cytopathic properties against sheep red cells coated with the crossreacting peptides found in Klebsiella and HLA-B27 sequences, (10) Klebsiella bacteria grow preferentially on carbohydrate substrates, and this could be used to decrease bowel bacteria which may lead to a reduction of inflammatory parameters, (11) post-pubertal hormonally-induced muscle mass leads to increased starch consumption and onset of AS. The use of “Popper Sequences” suggests that Klebsiella microbes are the probable causative agents of AS.