The replacement of lost body tissue in modern dairy cows usually takes place later in the same lactation, once milk output begins to decline. Individual cows may not regain all lost body energy in the first lactation leading to a greater deficit to be replenished in the second. This results in carry-over effects from one lactation to the next, on both yield and non-yield traits. The use of random regressions and Fourier series allows modelling of cyclical changes in a trait over seasons (Meyer, 2000) and could be used to model multiple lactation energy balance changes in dairy cows. Parameters of these cyclical functions might then be analysed for relationships with traits of economic importance such as health, fertility and survival. The objectives of this study were 1) to model phenotypic daily milk yield, fresh feed intake, liveweight and condition score using random regression techniques, 2) to combine daily estimates obtained in objective 1) into an overall energy balance for each day of the first three successive lactations, 3) to compare energy balance curves over three lactations and 4) to investigate the feasibility of modelling energy balance in the first three lactations using harmonic analysis.