We analyse TFP growth in the US business sector using a basic unobserved component model where trend growth follows a random walk and the noise is a first order autoregression. This is fitted using a Kalman-filter methodology. We find that trend TFP growth has declined steadily from 1.5 to 1.0 per cent per year over the past 50 years. Nevertheless recent trends are not a good guide to actual medium-term TFP growth. This exhibits substantial variations and is quite unpredictable.