Attempts to define the clinical significance of occult lymph node metastasis have yielded mixed results. We set out to quantify the influence on disease-free survival of occult lymph node metastasis in cases of esophageal or gastro-esophageal cancer previously staged as lymph node-negative by conventional H&E staining. We performed a systematic review and meta-analysis of observational studies published between 1966 and 2006 (identified through Medline and Embase). Twelve suitable cohort studies were identified. These studies suggest there is a significant (P < 0.001) association between occult lymph node metastasis and prognosis in cancer of the esophagus or esophago-gastric junction (pooled hazard ratio 3.16 with 95% confidence intervals of 2.25-4.42). We did not demonstrate study quality, number of nodes examined or number of lymph node sections examined to be significant sources of intertrial heterogeneity. Data from observational studies suggest that occult lymph node metastasis is an important prognostic factor in cancer of the esophagus or gastro-esophagus. Meta-analysis using individual patient data can now be justified.