Cellular blue nevus is an uncommon pigmented tumor in the conjunctiva, where it generally appears as a deep, circumscribed, pigmented conjunctival mass. We report a case of conjunctival blue nevus that clinically resembled primary acquired melanosis and gave rise to conjunctival melanoma. A 41-year-old man developed a diffuse pigmented mass in the inferior fornix of his left eye. Over a 20-year period, he noted slight progression of the pigment. Foci of epibulbar pigmentation were also present. The lesion resembled primary acquired melanosis. Excisional biopsy and adjuvant cryotherapy were performed. Histopathologic examination disclosed an intense infiltrate of heavily pigmented dendritic melanocytes with aggregates of less pigmented plump cells in the substantia propria. The conjunctival epithelium was normal. Malignant cellular features consistent with melanoma were observed in some foci. Cellular blue nevus of the conjunctiva can simulate primary acquired melanosis and can give rise to malignant melanoma. Arch Ophthalmol. 2000;118:1581-1584