The present study is the first complete presentation of the prognosis of all histological types of canine vascular tumours, using population-based data. A retrospective, longitudinal, population-based study of 420 dogs was carried out in 2001-2002 using updated histologically verified diagnoses of vascular tumours and tumour-like lesions recorded in Norwegian Canine Cancer Register (NCCR) 1990-1998. Data on clinical signs and outcome of the vascular tumour, and causes of death, were obtained from dog owners and veterinary practitioners by mailed questionnaires. The a priori set inclusion criteria required that both questionnaire forms were completed, which was the case for 162 of the 420 dogs (39%). A benign tumour was diagnosed in 61%, whereas 15% had a tumour of intermediate malignancy and 24% had a malignant one. Survival time was estimated by the method of Kaplan-Meier (product limit) survival plot, and the survival time for three histological diagnoses was compared by a log-rank test. Hazard ratios were estimated by Cox proportional hazards model. A p-value <0.05 was considered statistically significant. Median survival time of dogs with malignant vascular tumour was < or = one day, and the respective median survival times for intermediate malignant and benign types were 101 and > or = 1095 days (P<0.0001). The median survival time of dogs with splenic vascular tumours was > or = 1095 days, 110 days and < or = one day for benign, intermediate malignant and malignant tumour forms, respectively (P<0.0001). Since the histological diagnosis was made without any knowledge of the outcome, the present study confirms that the new histological criteria and methods employed have indeed a prognostic value. This is the first time it has been shown that a histopathological differentiation of malignant tumours into two well-defined subgroups differentiates the survival time correspondingly: dogs with less grave tumour diagnoses live 100-110 days longer than dogs with the obvious malignant tumours, if surgery alone is the treatment regime.