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(*** abridged ***) Context: The wings of Balmer lines are often used aseffective temperature diagnostics for late-type stars under the assumption theyform in local thermodynamic equilibrium.  Aims: Our goal is to investigate the non-LTE formation of Balmer lines inlate-type stellar atmospheres, to establish if the assumption of LTE isjustified. Furthermore, we aim to determine which collision processes areimportant for the problem; in particular, the role of collision processes withhydrogen atoms is investigated.  Method: A model hydrogen atom for non-LTE calculations has been constructedaccounting for various collision processes using the best available data fromthe literature. The processes included are inelastic collisions with electronsand hydrogen atoms, mutual neutralisation and Penning ionisation. Non-LTEcalculations are performed, and the relative importance of the collisionprocesses is investigated.  Results: Our calculations show electron collisions alone are not sufficientto establish LTE for the formation of Balmer line wings. The role of inelasticcollisions with neutral hydrogen is unclear. The available data for theseprocesses is of questionable quality, and different prescriptions for the ratecoefficents give significantly different results for the Balmer line wings.  Conclusions: Improved calculations or experimental data are needed forexcitation and, particularly, ionisation of hydrogen atoms in low-lying statesby hydrogen atom impact at near threshold energies. Until such data areavailable, the assumption of LTE for the formation of Balmer line wings inlate-type stars is questionable.Comment: Accepted for A&A; 12 pages, 11 figure

Non-LTE Balmer line formation in late-type spectra: effects of atomic processes involving hydrogen atoms