The influence of jet-flow and jet-jet interactions on the lift-off length of diesel jets are investigated in an optically accessible heavy-duty diesel engine. High-speed OH chemiluminescence imaging technique is employed to capture the transient evolution of the lift-off length up to its stabilization. The engine is operated at 1200 rpm and at a constant load of 5 bar IMEP. Decreasing the inter-jet spacing shortens the liftoff length of the jet. A strong interaction is also observed between the bulk in-cylinder gas temperature and the inter-jet spacing. The in-cylinder swirl level only has a limited influence on the final lift-off length position. Increasing the inter-jet spacing is found to reduce the magnitude of the cycle-to-cycle variations of the lift-off length. The measured lift-off lengths, at their stabilized positions, are combined with a previous database composed of variations of intake air temperature, motored top dead center density, fuel injection pressure, intake oxygen concentration and nozzle diameter. A non-linear regression model based on the power law form proposed by Siebers et al. is developed for describing the stabilized lift-off length of diesel jets as function of the ambient in-cylinder conditions, jet-flow and jet-jet interactions. The model has good accuracy. Although it is only validated for this particular engine environment it identifies the parameters that have the dominating effect on the lift-off length.