A theoretical analysis is presented of the Mapleson A, C and D breathing systems when used in spontaneous respiration. The influence of the respiratory pattern is explained diagrammatically. Simple equations are derived, predicting the fresh gas flow required to prevent rebreathing with different respiratory patterns. Further equations allow the degree of rebreathing caused by inadequate fresh gas flow to be quantified. These are used to examine the effects of different respiratory patterns on the efficiency of the three systems. It is demonstrated that the single most important determinant of efficiency is the duration of the expiratory pause. The nature of the inspiratory and expiratory waveforms is less important and the I:E ratio far less important. The analysis suggests that the Mapleson A system will always be the most efficient of the three systems. The Mapleson C system will be efficient if inspiration is long and the expiratory pause is minimal. The Mapleson D system will be efficient if the expiratory pause is sufficiently long.