A selected series of 669 primary malignant melanoma of the skin, stage I, was studied. The series included 86 lentigo maligna melanomas, 259 superficial spreading malignant melanomas, 194 nodular malignant melanomas and 130 unclassifiable malignant melanomas. The adjacent lymphocyte infiltration was graded and its prognostic value and its relation to the sex and age of the patient, tumour cell type, pigmentation, cellular atypia, mitotic count, depth of dermal invasion, tumour type and ulceration was studied. There was no significant relationship between lymphocyte response and sex and age of the patient and the tumour cell type. There was a highly significant relationship between a dense lymphocyte infiltration and superficial tumour invasion as far as the papillary-reticular interface in contrast to the weak response associated with deeper invasion. When only tumours with invasion of the papillary-reticular interface were considered, there was no significant relationship between lymphocyte infiltration and pigmentation, cellular atypia, mitotic count, tumour type and ulceration. At the same level of invasion there was no difference in prognosis in relation to the density of lymphocyte infiltration. Nodular malignant melanomas surrounded by a dense lymphocyte infiltration had a significantly worse prognosis than was associated with a simular lymphocyte response against the two other types of melanoma.