Clinico-pathological profiles were studied in adult and young Friesian cattle naturally infected with Theileria annulata in the Qassim Region, Saudi Arabia. Sixty-two clinical cases of T. annulata infection in adult and young Friesian cattle were diagnosed during the period from August 1999 to July 2000. Symptoms observed were marked fever, swelling of superficial lymph nodes, inappetance, tachycardia, dyspnoea and weakness. The most prominent gross pathological features were jaundice, petechial and ecchymotic haemorrhages involving mucosal and serosal surfaces of many organs as well as body fat. A number of young and adult Friesian cattle undergoing lethal T. annulata infection developed lymphoma-like lesions in a manner similar to that of T. parva. The main histological findings were necrosis and severe lymphocytic infiltration. The spleen, lymph nodes and Peyer's patches were devoid of typical lymph nodules.