Line mixing of the Na 3P–3D lines broadened by atomic hydrogen has been calculated in the impact approximation. The required S-matrix elements are calculated using a time-dependent close-coupling rectilinear-path approximation, the ab initio NaH potentials used by Leininger et al (2000 J. Phys. B: At. Mol. Opt. Phys. 33 1805) being employed. At energies of less than 25 000 cm⁻¹, the real and imaginary parts of the mixing cross sections were found to be about 50 and about 25 times smaller, respectively, than the broadening coefficients, while at about 10⁵ cm⁻¹, the real and imaginary parts were found to be smaller in magnitude by factors of about 200 and 10, respectively. On thermally averaging the cross sections, the line-shape parameters leading to mixing were found to be 50–100 times smaller than the corresponding broadening coefficients for the isolated lines. For a number density of 10¹⁶ – 10¹⁷ cm⁻³ and temperature of 6000 K, the mixed and unmixed line shapes were found to deviate by less than 3% over the range of frequencies for which the application of the impact approximation was valid.