Despite its very good specificity, serum neuron-specific enolase (SNE) cannot be relied upon to diagnose small cell carcinoma. However, this marker is of interest owing to the double correlation observed between it and tumoral extension and between its rapid elevation under chemotherapy and clinical response to chemotherapy. Repeated SNE assays help in determining this response when SNE returns to normal level, and in predicting the progression of the disease several weeks in advance when SNE levels increase. Early and later SNE assays therefore have prognostic value.