Objective: Fine particulate matter air pollution (PM 2.5) and extreme temperatures have both been associated with alterations in blood pressure (BP). However, few studies have evaluated their joint haemodynamic actions among individuals at high risk for cardiovascular events. Methods: We assessed the effects of short-term exposures during the prior week to ambient PM 2.5 and outdoor temperature levels on resting seated BP among 2078 patients enrolling into a cardiac rehabilitation programme at the University of Michigan (from 2003 to 2011) using multiple linear regression analyses adjusting for age, sex, BMI, ozone and the same-day alternate environmental factor (i.e. PM 2.5 or temperature). Results: Mean PM 2.5 and temperature levels were 12.6±8.2μg/m 3 and 10.3±10.4°C, respectively. Each standard deviation elevation in PM 2.5 concentration during lag days 4-6 was associated with significant increases in SBP (2.1-3.5mmHg) and DBP (1.7-1.8mmHg). Conversely, higher temperature levels (per 10.4°C) during lag days 4-6 were associated with reductions in both SBP (-3.6 to -2.3mmHg) and DBP (-2.5 to -1.8mmHg). There was little evidence for consistent effect modification by other covariates (e.g. demographics, seasons, medication usage). Conclusion: Short-term exposures to PM 2.5, even at low concentrations within current air quality standards, are associated with significant increases in BP. Contrarily, higher ambient temperatures prompt the opposite haemodynamic effect. These findings demonstrate that both ubiquitous environmental exposures have clinically meaningful effects on resting BP among high-risk cardiac patients.