Forty-one cases of conjunctival primary acquired melanosis (PAM) were studied to determine the frequency of progression to malignant melanoma and to establish prognostic parameters for progression to melanoma. Two subdivisions were identified: lesions with cytologically atypical melanocytes (PAM with atypia, 28 lesions, 68.3 per cent) and those lacking cytologically atypical melanocytes (PAM without atypia, 13 lesions, 31.7 per cent). None of the lesions of PAM without atypia progressed to melanoma. Thirteen of the 28 lesions of PAM with atypia (46.4 per cent) progressed to melanoma. Progression to melanoma was more frequent in the lesions of PAM with atypia if basilar hyperplasia was not the dominant histologic pattern (90 per cent progression, P = 0.02) or if any epithelioid cells were present (75 per cent progression, P = 0.02). It was not possible to determine which lesions were atypical on the basis of clinical appearance. Lesions at risk for the development of melanoma should be totally extirpated.