The aim of the present investigation was to study the variation in interdental forces between mandibular canines and lateral incisors of 19 volunteers (9 males and 10 females) aged 20-26 years for four configurations (mandible open/closed and left/right side). These forces were derived by pulling a stainless steel matrix strip between these teeth, six times per configuration, and registering the time variation with a high-resolution transducer. The repeated median smoothing algorithm was applied to find the maximum of each curve and a bootstrap method estimated the 95 per cent confidence intervals (CIs) for all 76 configurations. Seventy-six per cent of all paired force differences were found to be significant. Asymmetry phenomena were observed: the interdental forces differed significantly between the left and right sides and also between the open and closed position of the mandible. The interdental forces (4-21 N) showed a pattern modulated by volunteer-specific features: in 91 per cent of the configurations, the interdental forces were larger when the mouth was open. This observed pattern contributes to the instability observed in clinical practice, thus necessitating permanent fixed lower retainer wear.