A study was made of 45 patients with diaphragmatic herniation after penetrating trauma. In 29 the diagnosis was established during the first admission (early presentation) and in 16 during a subsequent admission (delayed presentation). The mortality rate in the early presentation group was 3 per cent compared with 25 per cent in the delayed presentation group. The presence of gangrenous or perforated abdominal viscus in the chest cavity was the single most common and severe aggravating factor. The need for diagnosis of diaphragmatic herniation during the initial admission is emphasized. As isolated diaphragmatic injuries provide few helpful clinical features to aid diagnosis, appropriate investigations and good follow-up are of paramount importance in preventing late herniation of intra-abdominal viscera through a penetrating diaphragmatic injury.