1. This manuscript describes two different strategies to progress from the clinical assessment of patients to the identification of disease-causing mutations. In the first disease, recognition of a metabolic abnormality allowed direct molecular analysis of the causal gene. In contrast, localization of the second disease gene by linkage analysis was critical to implicate a gene with a previously unsuspected disease role. 2. Two sisters with chronic respiratory disease and recurrent infections were identified as the first cases of adult onset immunodeficiency due to adenosine deaminase deficiency. Autosomal recessive inheritance of two mutations in the adenosine deaminase gene was demonstrated. Enzyme replacement therapy improved the patients' immunological and clinical status. 3. Individuals with pulmonary arteriovenous malformations were used to identify families with hereditary haemorrhagic telangiectasia (HHT, Rendu-Osler-Weber Syndrome). Linkage studies mapped the HHT disease gene in some families to chromosome 9, and demonstrated genetic heterogeneity. The chromosome 9 disease interval was refined, and several candidate genes were assessed. Following the first description of disease-segregating mutations, a complete analysis of the endoglin gene (which encodes an endothelial cell transforming growth factor-beta receptor) identified seven novel mutations. Two mutations did not produce mutant mRNA, and disease severity was comparable between families, indicating that HHT results from stoichiometric insufficiency of endoglin. 4. Each study has implications extending beyond the relatively rare disease analysed. The adenosine-deaminase-deficient patients highlight a treatable cause of HIV-negative CD4+ lymphopenia in adults, perhaps accounting for further cases of 'non-HIV AIDS'. The HHT studies have illuminated a novel area of vascular pathophysiology, with potential relevance to further disease states.