Preterm and term infants often become sick and need different forms of medical or surgical interventions. During sickness, infants’ appetite decreases which results in nutritional problems. The consequences of inadequate nutrition can be changes in growth. The options to measure growth during short periods, from a few days up to one week, in unhealthy preterm infants have been limited to body weight. Weight can be hard to estimate because of intensive care limiting our possibilities to handle the infant. However, increasing weight is not always associated with good growth/health and in contrast it can be associated with decreased kidney function or given pharmacy. Knemometry is a validated and reliable method for measuring the distance from the heel to the upper side of the 90 degree-flexed knee, the knee-heel length (KHL). The daily increase in KHL is about 0.5 mm. The precision of the instrument is 0.05 mm, and therefore daily measurements can be done. The aims of this study were to 1) estimate the KHL in healthy newborns and 2) to compare KHL in unhealthy preterm-term infants up to ten months of life. The study group included about fifty term and preterm healthy (reference group) and sick infants (case group). A validated mini-knemometer produced by the Medical Technical Department at Aarhus University, Denmark was used. At each measurement, five estimations were done consecutively, and the mean value and standard deviation (SD) were estimated. The total number of occasions of analysis for each child differed. The period of analysis varied from a few weeks up to 10 months of age. A reference curve for KHL from birth up to 10 months, adjusted for preterm births, is presented. The curve shows a continuous decrease in linear growth. Four selected cases with different etiologies and outcomes are illustrated and marked in separate reference curves. Knemometry is a simple, painless, validated and reliable method to measure the linear growth rate (knee-heel length) in preterm-term infants. A normal KHL progress is associated with good health.