PURPOSE: The monitoring of serum electrolyte and creatinine concentrations in patients treated with antihypertensive therapy is recommended. We wished to examine the relationship between laboratory monitoring and adverse patient outcomes. METHODS: We carried out a retrospective cohort study using the General Practice Research Database (GPRD). Patients aged 18 years or older with newly diagnosed hypertension and prescribed a single antihypertensive agent were included. Monitoring was defined as any laboratory test for serum electrolyte and creatinine (or urea) concentrations within 6 months of starting treatment. RESULTS: We identified 74 096 patients who were newly diagnosed with hypertension and prescribed a single antihypertensive agent. Twenty six thousand nine hundred forty six (36.4%) patients had any biochemical laboratory measurement within 6 months. Three hundred ten patients (0.4%) died, 1451 (2%) were admitted to hospital at least once and 29 749 (40.2%) discontinued their first course of antihypertensive treatment within 6 months. Patients were more likely to be admitted to hospital if their biochemistry had been monitored after beginning treatment (adjusted hazard ratio (HR) 1.37; 95%CI 1.21-1.55). They were also marginally more likely to discontinue treatment (adjusted HR 1.04; 95%CI 1.02-1.07). They were not significantly more likely to die (adjusted HR 1.21; 95%CI 0.87-1.67). CONCLUSIONS: Biochemical testing at baseline and monitoring after starting treatment is often omitted in newly diagnosed hypertensive patients. Those patients who are monitored are more likely to be admitted to hospital and to discontinue initial antihypertensive therapy, but not to die. Many biochemical adverse drug reactions are found only by laboratory monitoring.