This thesis describes the long-term follow up of a birth cohort of boxer dogs to study disease incidence, mortality, as well as genetic and environmental risk factors in a purebred dog population in The Netherlands. During the last decades, both the number of genetic disorders and the disease rate of these disorders seemed to increase. Therefore, special attention was given to presumed genetic disorders. Heritability in this thesis was estimated using a proportional hazard model based on the Weibull distribution as implemented in the Survival Kit software. Parametric survival models were developed to identify the influence of potential risk factors. Risk factors included individual data and litter data. Between November 1999 and March 2004 a total of 1863 boxer dogs from 388 litters, all born between January 1994 and February 1995 were monitored. Data was collected by interviewing the dog-owners through self-completion (mail) questionnaires, which were sent every 6 months. At the end of the study 674 boxer dogs had survived up to that point, 530 dogs had died (including euthanasia). Survival analyses, accounting for loss to follow up, resulted in a mortality rate of 45% of the cohort of boxer dogs. The highest disease incidence in this boxer dog population was attributable to hind limb lameness of which cranial cruciate ligament rupture (22%) and canine hip dysplasia (8.5%) were most frequently diagnosed. When the box-floor was covered with slippery material, the risk of cCHD increased 1.6 times, and the risk of cCHD doubled in litters with high preweaning mortality. Castration, =6 months prior to cCHD diagnosis, increased the risk of cCHD 1.5 times. The general risk of boxer dogs developing cCHD and CCL rupture after 5 years of age was 1.8 and 2.4 times higher than before the age of 5, respectively. The effective heritability of cCHD was 11% and of CCL rupture was 16%. We described tumor and cancer incidence. Classification of skin tumors showed a high incidence of mesenchymal tumors (88%). The risk rate of ...