The present invention provides a method of diagnosing or predicting susceptibility to a clinical subtype of Crohn's disease characterized by fibrostenosing disease by determining the presence or absence in an individual of a fibrostenosis-predisposing allele linked to a NOD2/CARD15 locus, where the presence of the fibrostenosis-predisposing allele is diagnostic of or predictive of susceptibility to the clinical subtype of Crohn's disease characterized by fibrostenosing disease. In a method of the invention, the clinical subtype of Crohn's disease can be, for example, characterized by fibrostenosing disease independent of small bowel involvement. The invention also provides a method of optimizing therapy in an individual by determining the presence or absence in the individual of a fibrostenosis-predisposing allele linked to a NOD2/CARD15 locus, diagnosing individuals in which the fibrostenosis-predisposing allele is present as having a fibrostenosing subtype of Crohn's disease, and treating the individual having a fibrostenosing subtype of Crohn's disease based on the diagnosis.