A systematic meta-analysis of observational studies of melanoma and one of the most important risk factors, the number of naevi, was conducted in order to clarify aspects of the aetiology of this disease. Following a systematic literature search, relative risks (RRs) were extracted from 46 studies published before September 2002. Dose-response random effects models were used to obtain pooled estimates. Sub-group analysis and meta-regression were carried out to explore sources of between-study variation and bias. Sensitivity analyses investigated the reliability of the results and any publication bias. Number of common naevi was confirmed an important risk factor with a substantially increased risk associated with the presence of 101-120 naevi compared with <15 (pooled Relative Risk (RR) = 6.89; 95% Confidential Interval (CI): 4.63, 10.25) as was the number of atypical naevi (RR = 6.36 95%; CI: 3.80, 10.33; for 5 versus 0). The type of study and source of cases and controls were two study characteristics that significantly influenced the estimates. Case-control studies, in particular when the hospital was the source for cases or controls, appeared to present much lower and more precise estimates than cohort studies.