Comprehensive (p,rho,T) measurements on pure ethylene have been carried out in the single-phase region (678 values) and along the entire (vapour + liquid) coexistence curve (185 values). The results reported in this first paper cover the homogeneous gas and liquid region in the temperature range from 105 K to 340 K at pressures up to 12 MPa. The total uncertainty of the measurements in density rho is < (1.5 . 10(-4).rho to 2.0 . 10(-4).rho) with the exception of the temperature range from 281 K to 305 K (T-c = 282.350 K) at densities between 120 kg . m(-3) and 280 kg . m(-3). There, the total uncertainty in pressure is < (1.0 . 10(-4). p to 1.5 . 10(-4). p). Values for the second and third virial coefficients have been determined from the measurements in the homogeneous gas region of ethylene. The experimental results have been compared with the results of previous workers.