Objective: To determine the clinicopathologic features and the disease course of patients with follicular mycosis fungoides (MF).

A review of sixty-four biopsies from sixteen patients with lymphomatoid papulosis revealed two characteristic histological types (type A and type B), which also had a different clinical behaviour. In lymphomatoid papulosis type A lesions, four histological patterns corresponding with the age of the lesion could be distinguished. Such a relationship was not found in type B lesions. The findings of transitional forms in some biopsy specimens, showing histological features of both type A and type B, and the presence of both types in different but concurrent lesions, suggests that these two types are not different entities but rather represent the ends of a spectrum. At least two different populations of atypical cells can be distinguished in lymphomatoid papulosis. Apart from the atypical cerebriform mononuclear cells, which are T-lymphocytic in origin and predominant in type B lesions, large atypical cells with vesicular nuclei, prominent nucleoli and abundant cytoplasm are found, particularly in type A lesions. Preliminary immunohistochemical and cytochemical investigations suggest that these cells are not lymphoid in origin, but are related to the Langerhans cell series.

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Some cutaneous melanocytic lesions are notoriously difficult to diagnose by histopathology. For that reason, the Pathology Panel of the Dutch Melanoma Working Party was instituted and is regularly approached to provide an expert opinion on problem cases. In order to identify the most common diagnostic problems, 1069 consecutive referral cases of submitted lesions (1992 to 1994 inclusive) were analysed. About 60 per cent of the requests came from small laboratories, with up to three consultant pathologists. Two‐thirds of the lesions reviewed concerned women and nearly 50 per cent of the patients were 30 years of age or younger. In 8 per cent of the cases, the referring pathologists felt unable to make a confident diagnosis; in 14 per cent, melanoma was suspected; and in 12 per cent, a differential diagnosis only had been formulated. The panel felt able to provide an unequivocal diagnosis in 93 per cent of the requests. Of the 158 lesions classified as ‘invasive melanoma’ by the referring pathologists, 22 were considered to be benign by the panel. Conversely, 108 invasive melanomas (panel diagnosis) had originally been considered as benign lesions, dysplastic naevi or melanoma in situ. These high numbers of discordancies reflect the intrinsic difficulty of the differential diagnoses in this selected material submitted to the panel. Diagnostic difficulties were most often encountered with Spitz naevi and dysplastic naevi. Although the rate of overdiagnosis and underdiagnosis is quite high, in the majority of cases the diagnosis of the referring pathologist matched the diagnosis of the panel. This may reflect a proper awareness of difficult melanocytic lesions in pathology practice. The activities of the Pathology Panel of the Dutch Melanoma Working Party contribute to the improvement of the quality of diagnosis in cutaneous melanocytic lesions, as they increase the number of unequivocal diagnoses and reduce the number of incorrect diagnoses. On the basis of the systematic comparison of the diagnosis by the referring pathologist and the panel, postgraduate teaching and quality control can be more focused on specific diagnostic problems. © 1997 John Wiley & Sons, Ltd.