Five growth hormone deficient children, aged 5.3 to 12.6 yrs, were measured regularly once or twice weekly by knemometry, a novel and noninvasive technique of accurate lower leg length measurement. The total period of observation was 40 months in the 5 children. During this time all children received replacement therapy with extractive human pituitary growth hormone 12 IU/m2/week by daily s.c. injections. 11 intermittent infectious illnesses occurred within the observation period of 40 months. During the infectious diseases a significant decrease of the mean lower leg growth velocity down to -0.012 mm/day was observed. During the following convalescent period (14 days) mean lower leg growth velocity rose up to +0.107 mm/day (p less than 0.001). Growth hormone substitution was not changed throughout the period of observation.