Lack of adherence to long-tenn therapies has been recognised for many years as a major problem, commonly undennining the effectiveness of medical care. The main objectives of this thesis were to evaluate the effect of nurse-led adherence support compared with usual care on adherence, blood pressure and costs, and to compare a newly developed adherence self-report tool with electronic monitoring. A total of 245 uncontrolled hypertensive patients with a diagnosis of essential hypertension were recruited in 22 general practices in Avon. A validation study of an adherence self-report tool compared with electronic monitoring showed that self-report can predict timing compliance at higher levels of adherence. More research is needed on the usefulness of this tool in day-to-day practice and in a more representative study sample. The main study of this thesis, the RCT, compared a nurse-led adherence support consultation followed by a re-inforcement appointment two months later with usual care alone. The main outcomes in this RCT were adherence to blood pressure lowering medication ('timing compliance'), systolic and diastolic blood pressure, and costs. There was no evidence of an effect of nurse-led adherence support on timing compliance (difference between means: -1.0,95% CI: -5.1 to 3.1, p=0.63), systolic blood pressure (difference between means: -2.7 mmHg, 95% CI: -7.2 to 1.8, p=0.24) or diastolic blood pressure (0.2, 95% CI: -1.9 to 2.3, p=0.85). With respect to the evaluation of the adherence self-report tool, there is strong evidence that a reduction of one level of self-reported adherence is associated with a decrease in timing compliance of around 5% (p=0.0004). In conclusion, nurse-led adherence support was no more effective than usual care in terms of increasing adherence or reducing blood pressure. Baseline adherence levels were high in both comparison groups, leaving little room for further improvement. In the few participants who did have medication problems, the intervention appeared to be successful, but further research is needed to consolidate this finding.