We investigate the influence of turbulent inlet boundary conditions on indoor air-flow characteristics and pollutant dispersion in Reynolds-averaged Navier-Stokes (RANS) simulations of an indoor ventilated enclosure at a transitional slot Reynolds number. A benchmark ventilation case is considered – for comparison experimental PIV data are available. Two turbulence closure models are included in the study, i.e., a low-Reynolds-number k–ɛ model, and the SST k–ω model. When looking at velocity fields, we find that the influence of turbulent length scales at the inlet boundary on the indoor flow field is small. The influence of turbulence intensity (ranging between 2% and 30%) is considerably larger, in particular affecting the separation point of the inlet jet along the top wall. When further investigating the effect of turbulent conditions at the inlet on pollutant dispersion for a hypothetical indoor contaminant release case, we find that variations of inlet turbulent length scales lead to differences in pollutant concentration of up to 20%. Variations due to changes in inlet turbulent intensity lead to differences up to a factor 2. These findings strongly emphasize the importance of imposing realistic boundary conditions for turbulence models, refuting the common working hypothesis, often used in the indoor-ventilation literature, that varying these conditions has negligible impact on simulation results.