Outbreaks of Marek's disease (MD) were diagnosed in two flocks from the same company. Clinical signs, mainly blindness (>90%), but also depression, mild paralysis, and 11 to 12% mortality by 20 weeks of age were observed. MD virus, serotype 1 was isolated. The isolates were designated NC-1 (flock 1) and NC-2 (flock 2). Challenge experiments were conducted with these isolates and with two reference MD virus strains (JM/102W and Md5) in unvaccinated, turkey herpesvirus- (HVT) vaccinated and bivalent- (HVT and SB-1) vaccinated chickens. Blindness, gross ocular lesions and tumour formation were observed in a high proportion of all groups challenged with NC-1 and NC-2 when compared with chickens challenged with JM/102W and Md5. In chickens challenged with isolates NC-1 and NC-2, corneal changes included oedema, midstromal cellular infiltration consisting of macrophages, lymphocytes, plasma cells and lesser numbers of heterophils, collagen degeneration and keratic precipitates consisting primarily of macrophages covering the central endothelium. Eosinophilic intranuclear inclusion bodies were present in mononuclear cells infiltrating the cornea. Changes in the uveal tract consisted of inflammatory cell infiltrates similar to those present in the cornea. Retinal lesions included disruption of the retinal pigmented epithelium, inflammatory cell infiltration in the subretinal space, photoreceptor degeneration and in severely affected eyes, necrosis of retinal cellular elements. Pecten changes consisted of necrosis and mononuclear cell infiltration. Intranuclear inclusion bodies were abundantly present in cells of the retina's ganglion and inner nuclear cell layers. The unusual clinical manifestation of MD, the unusual tropism and virulence of NC-1 and NC-2 for ocular tissues and the incomplete protection afforded by conventional vaccination suggest that these isolates may be new pathotypes.