Delayed diagnosis is a critical patient safety concern. This cohort study, consisting of 3,500 patients with tuberculosis (TB) over a 17-year period, found that more than three-quarters of patients experienced at least one missed opportunity for a diagnosis in the year before they were finally diagnosed with TB. The average duration of the diagnostic delay was nearly 32 days. Missed opportunities occurred most commonly in outpatient settings. A previous WebM&M commentary discusses patient harm resulting from a missed TB diagnosis.