Summary.A study is made of the duration of physiological icterus neonatorum in a group of apparently healthy children. The investigation is based on 946 children in all, 476 boys and 470 girls. Of these, 607 (64.16 ± 1.56%) exhibited jaundice. Jaundice of longer duration than 48 hours and of rather distinct character occurred in a total of 291 children (30.70 ± 1.50%). Statistically, this rather pronounced jaundice was more common in children with a low birth‐weight than in children with normal birth‐weight. Among children with a birth‐weight below 2500 grams it thus occurred in 82.05 ± 6.15% of cases, among those with a birth‐weight above 2500 grams in 28.56 ± 1.49%Jaundice appears somewhat earlier in children with a low birth‐weight than in those with normal birth‐weightThe average duration of physiological jaundice lasting in general more than 48 hours has beeen 11.37 ± 0.36 days in the present material. Children with a low birth‐weight show, on an average, a longer duration of jaundice than children with birth‐weights above 3000 gramsIcterus neonatorum prolongatus, i.e. icterus with a duration of at least 14 days, occurred in 80 cases out of the 946, which gives a frequency of 8.46 ± 0.90%. Here, too, the frequency is incomparably highest in children with a birth‐weight below 2500 grams (48.72 ± 8.00%). In children with birth‐weights above 2500 grams the corresponding frequency was 6.73 ± 0.83%. Among children with a low birth‐weight, below 2500 grams, jaundice lasted between 14 and 20 days in 7 cases and between 20 and 30 days in the remaining 12. Infants with birth‐weights above 2500 grams showed a duration of between 14 and 20 days in 35 cases, of between 20 and 30 days in 24 cases, and of more than 30 days in 2 instances. The longest duration observed was 37 daysChildren having a distinct and rather long‐standing jaundice showed a, statistically verified, greater initial fall in weight than a control series with normal birth‐weight and without visible jaundiceA study of the behaviour of the prothrombin index after prophylactic administration of vitamin K on the second or third day of life showed no difference between eases with or without