Production historics of four waterfloods, each with several infill wells, were examined to identify conditions favourable to infill drilling in sandstone redervoirs containing heavy/medium oils under waterflood. The dominant role of reservoir heterogeneity was inferred from the nature of performance of newer and older infill wells. Due to these heterogeneities, some of the mobile oil contained within the reservoir is not drained by the older wells, but infills wells could drain a portion (up to 25%) of this oil. Based on the cases studied, conditions favourable for infill wells were: a) Current oil rates of older wells greater than 3 m(3)/d and watercut less than 75%. b) Original Oil-in-Place (OOIP)/well for the reduced spacing greater than 80 E3m(3) and total recovery factor (primary + waterflood) greater than 25%. It is recommended that these criteria be validated by further technical and economic studies.