WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: Guidelines recommend biochemical monitoring of patients treated with antihypertensive agents, although there is little primary evidence for these recommendations. WHAT THIS STUDY ADDS: Patients treated for hypertension in general practice often have no biochemical tests before, or in the 6 months after, starting drug treatment. AIMS: Guidelines on the management of hypertension have recommended baseline testing of serum electrolyte and creatinine concentrations before treatment since the 1990s. We wished to examine the extent of laboratory monitoring in patients with newly diagnosed hypertension and newly treated with antihypertensive drugs. METHODS: We carried out a retrospective analysis of 74,096 patients in the General Practice Research Database (GPRD) aged 18 years and older with newly diagnosed hypertension and prescribed a single antihypertensive agent. We determined the number of patients with a laboratory test for serum electrolyte and creatinine (or urea) concentrations prior to the first antihypertensive drug prescription and in the 6 months after and patient factors associated with subsequent monitoring. RESULTS: Thirty-four thousand nine hundred and forty-seven patients (47%) had at least one biochemical test in the 12 months prior to beginning antihypertensive treatment, and 26,946 (36%) had at least one biochemical monitoring test in the 6 months after beginning antihypertensive treatment. Thirteen thousand five hundred and four (18%) had both baseline and monitoring tests. Baseline tests were normal in 11,671 patients (86%), of whom 10,213 (88%) had normal tests at first monitoring. Monitoring was significantly more likely in patients treated with ACE inhibitors than thiazides (adjusted OR 1.90; 95% CI 1.80, 2.00), older patients (adjusted OR 1.23; 95% CI 1.11, 1.36) [individuals aged 80-89 years compared with <40 years], and patients with diabetes mellitus (adjusted OR 2.03; 95% CI 1.91, 2.16). CONCLUSION: Biochemical testing at baseline and monitoring after starting treatment is often omitted in newly diagnosed hypertensive patients. When both are tested, one in eight normal results becomes abnormal.