OBJECTIVES: For patients with hypertension, successful pharmacologic treatment depends on the efficacy of the drug as well as its tolerability. The purpose of this study was to develop and test a rating system that characterizes an agent s effectiveness in terms of clinical efficacy relative to patient perception of tolerability. The Perceptual Effectiveness Index (PEI) is a 4-point ordinal scale, where 0 indicates the agent is not effective in controlling blood pressure, regardless of side effects, and 1 indicates optimal effectiveness, i.e., blood pressure control with minimal to no side effects. METHODS: The PEI was tested in 243 patients participating in an 8-week, multicenter, double-blind trial comparing the efficacy, safety and tolerability of candesartan cilexetil versus amlodipine. RESULTS: Mean age of the sample was 53 years (+ 12); 54% were male. Of the 202 who achieved control of their hypertension, 62 (31%) experienced side effects that were classified as moderate to high impact; 33 (16%) experienced side effects with moderate impact, and 29 (14%) fell into the high impact group. There was a statistically significant relationship between these PEI categories and global HRQL (p<0.001), utility (visual analog scale-transformed) (p<0.001), and satisfaction with treatment (p<0.001). Although subjective compliance declined as PEI score declined, the relationship was not statistically significant. A significant relationship was found between treatment and PEI score (p<0.05), with more patients in the candesartan cilexetil group achieving control with low side-effects and fewer falling into the high impact category. CONCLUSION: Results suggest the PEI is a useful summary indicator of the efficacy of a given agent relative to the patient s perception of tolerability. Further study of the predictive value of this index in terms of drug-switching and long-term compliance in hypertension and the usefulness of this type of system for other products and indications is warranted.