In high intensity proton linacs, beam halo can be observed as early as the very first stages of acceleration. The inital population of halo particles found in the output distribution of the RFQ, increases in chopper lines, which interrupt the regular focusing pattern of RFQ and DTL. The resulting emittance growth and halo development contributes to beam loss in the low‐energy accelerator sections and partly defines the beam quality in the high‐energy sections. Using the examples of the ESS and Linac4 (CERN) front ends (RFQ to 20 MeV) we investigate emittance growth and beam halo due to statistically distributed field and gradient errors. © 2003 American Institute of Physics