Background and objectives: Secondary analysis of the Hemodialysis Study showed that serum β2-microglobulin levels predicted all-cause mortality and that high-flux dialysis was associated with decreased cardiac deaths in hemodialysis patients. This study examined the association of serum β2-microglobulin levels and dialyzer β2-microglobulin kinetics with the two most common causes of deaths: Cardiac and infectious diseases. Cox regression analyses were performed to relate cardiac or infectious deaths to cumulative mean follow-up predialysis serum β 2-microglobulin levels while controlling for baseline demographics, comorbidity, residual kidney function, and dialysis-related variables. Results: The cohort of 1813 patients experienced 180 infectious deaths and 315 cardiac deaths. The adjusted hazard ratio for infectious death was 1.21 (95% confidence interval 1.07 to 1.37) per 10-mg/L increase in β2-Microglobulin. This association was independent of the prestudy years on dialysis. In contrast, the association between serum β2-microglobulin level and cardiac death was not statistically significant. In similar regression models, higher cumulative mean Kt/V of β 2-Microglobulin was not significantly associated with either infectious or cardiac mortality in the full cohort but exhibited trends suggesting an association with lower infectious mortality (relative risk 0.93; 95% confidence interval 0.86 to 1.01, for each 0.1-U increase in β2-microglobulin Kt/V) and lower cardiac mortality (relative risk 0.93; 95% confidence interval 0.87 to 1.00) in the subgroup with >3.7 prestudy years of dialysis. Conclusions: These results generally support the notion that middle molecules are associated with systemic toxicity and that their accumulation predisposes dialysis patients to infectious deaths, independent of the duration of maintenance dialysis. Copyright © 2008 by the American Society of Nephrology.