Diaphragmatic hernias of the Morgagni type are generally thought to be asymptomatic in adults. This traditional assumption led to a delay in diagnosing a Morgagni hernia as the cause of acute respiratory distress in a chronic schizophrenic man. While Morgagni hernias are usually considered to be long-standing, we present radiological evidence of an acutely expanding hernia. The patient's symptoms were relieved by surgical repair of the hernia. We advise caution before dismissing Morgagni hernias in adults as being long-standing and clinically insignificant.