We reviewed 33 cases of Spitz naevus and 19 of malignant melanoma in patients aged 20 years or less who were evaluated at our institution from 1950 to 1975 and followed-up for up to 32 years. The histological findings were studied prior to review of clinical data. There were no malignant melanomas in patients less than 9 years old; almost half (15/33) of the Spitz naevi were in this age group. Among the 25 histological criteria evaluated in the 52 lesions, the most striking differences between malignant melanomas and Spitz naevi were a higher degree of pagetoid spread, cellular pleomorphism, nuclear hyperchromasia and mitotic activity in the malignant melanomas, and a more prominent spindle cell component in Spitz naevi. We stress the importance of cytological as well as architectural criteria in distinguishing between Spitz naevi and malignant melanomas and emphasize the pitfalls that may be encountered because of overlap in histological features between the two groups.