Positron emission tomography with 18-fluorodeoxyglucose (FDG-PET) seems to be superior to computed tomography (CT) in staging the mediastinum in patients with non-small-cell lung cancer (NSCLC). However, recent results suggest that FDG-PET performance characteristics are conditional for nodal size as shown by CT: FDG-PET is more sensitive but less specific with lymph node enlargement on CT. The association between size and the probability of malignancy needs to be known to predict the post-test probabilities after PET, and finally, stratify patients for mediastinoscopy or thoracotomy depending on the PET and CT results. Therefore, we performed a meta-analysis of available studies reporting on the prevalence of metastatic involvement for different size categories of enlarged lymph nodes in patients with NSCLC and were able to include 14 studies. The prevalence of metastatic involvement and conditional test performance of CT and FDG-PET were calculated for lymph nodes measuring 10-15 mm, 16-20 mm and >20 mm. We found a post-test probability for N2 disease of 5% for lymph nodes measuring 10-15 mm on CT in patients with a negative FDG-PET result, suggesting that these patients should be planned for thoracotomy because the yield of mediastinoscopy will be extremely low. For patients with lymph nodes measuring > or =16 mm on CT and a negative FDG-PET result a post-test probability for N2 disease of 21% was found, suggesting that these patients should be planned for mediastinoscopy prior to possible thoracotomy to prevent too many unnecessary thoracotomies in this subset.