Circumscribed and diffuse pleural thickening (PT), the most common radiographic manifestations of asbestos-related disease, have different pathogeneses, differential diagnostic criteria, and effects on pulmonary function. The important distinction between the two disorders is demonstrated by six patients in whom typical circumscribed PT was followed by diffuse PT in the same hemithorax. The diffuse PT came on 18-47 (mean 32.3) years after first exposure to asbestos. In four patients, clinically evident pleural effusions preceded the diffuse PT; the latter left all with moderate to severe restrictive ventilatory impairment. Patients with stable pleural plaques are at risk for diffuse PT and consequent greater ventilatory impairment, even though they are many years from first exposure to asbestos.