Recent experiments in ultrafast physics have established the importance of above-threshold ionization (ATI) experiments in measuring and controlling the carrier-envelope phase (CEP) of few-cycle laser pulses. We have performed an investigation of atomic hydrogen subjected to intense CEP-stable few-cycle laser pulses. The experimental ATI spectra have been compared to predictions from an ab initio numerical solution of the time-dependent Schr椩nger equation in three dimensions. Good agreement between experiment and theory has been achieved without using any free fit parameters. Our results provide an important step towards obtaining calibrated reference data for a direct comparison of ATI electron yields for a range of gas species and experimental conditions.