We sought information worldwide on mortality according to assigned treatment in all randomized trials that began before 1985 of adjuvant tamoxifen or cytotoxic therapy for early breast cancer (with or without regional lymph-node involvement). Coverage was reasonably complete for most countries. In 28 trials of tamoxifen nearly 4000 of 16,513 women had died, and in 40 chemotherapy trials slightly more than 4000 of 13,442 women had died. The 8106 deaths were approximately evenly distributed over years 1, 2, 3, 4, and 5+ of follow-up, with little useful information beyond year 5. Systematic overviews of the results of these trials demonstrated reductions in mortality due to treatment that were significant when tamoxifen was compared with no tamoxifen (P less than 0.0001), any chemotherapy with no chemotherapy (P = 0.003), and polychemotherapy with single-agent chemotherapy (P = 0.001). In tamoxifen trials, there was a clear reduction in mortality only among women 50 or older, for whom assignment to tamoxifen reduced the annual odds of death during the first five years by about one fifth. In chemotherapy trials there was a clear reduction only among women under 50, for whom assignment to polychemotherapy reduced the annual odds of death during the first five years by about one quarter. Direct comparisons showed that combination chemotherapy was significantly more effective than single-agent therapy, but suggested that administration of chemotherapy for 8 to 24 months may offer no survival advantage over administration of the same chemotherapy for 4 to 6 months. Because it involved several thousand women, this overview was able to demonstrate particularly clearly that both tamoxifen and cytotoxic therapy can reduce five-year mortality.