Airway obstruction is a relative contraindication to diving. Dive candidates are assessed clinically, and lung function tests evaluate variables such as forced vital capacity (FVC), forced expiratory volume in one second (FEV1) and the FEV1/FVC ratio. A small number of individuals have a normal FEV1, but a disproportionately large lung capacity, or pulmonary dysanapsis. These individuals have a decreased FEV1/FVC ratio, suggesting airway obstruction, which may affect their dive medical assessments. Three cases of pulmonary dysanapsis presented for fitness-to-dive assessment. Case 1, a 29-year-old male had an FEV1: 3.52 L (85% predicted), FVC: 5.31 L (108.5% predicted), giving a FEV1/FVC of 66%. Case 2, a 25-year-old male with an FEV1: 4.55 L (95% predicted), FVC: 7.0 L (121% predicted) and a FEV1/FVC of 66%. Albuterol produced an FEV1 increase of 11%, but his hypertonic saline challenge was negative. Case 3, a 61-year-old man had an FEV1: 3.49 L (126% predicted), FVC: 7.06 L (216% predicted), and a FEV1/FVC of 49%. This report highlights pulmonary dysanapsis which may be confused with obstructive airway disease and applicants deemed unfit to dive. While pulmonary dysanapsis may increase the risk of airway hyperresponsiveness, there is no evidence of an association with diving-related pulmonary barotrauma.