Controlling for socioeconomic and geographic factors, under-5 mortality (5q0) in developing countrieshas been declining at about 2.7% per year, a high rate of ‘technical progress’. This paper adduces the-oretical and empirical reasons for rejecting the usual specification of homogeneous technical progressacross countries and uses a panel of 95 developing countries for the period 1970–2000 to explore theconsequences of heterogeneity. Allowing country-specific rates of technical progress sharply reduces theestimated income elasticity of 5q0 and points to country variation in technical progress as the principalsource of the (large) cross-country variation in 5q0 decline. Education levels and physician coverage alsocontribute and are less affected than income of allowing country variation in technical progress. Thepaper concludes by decomposing 1970–2000 5q0 decline into its different sources for each country.