Hospital length of stay (LOS) of Japanese inpatients is extraordinarily long, partly because of the nature of the health insurance system in Japan. We investigated factors that affect the LOS in Japan, taking pediatric nephrosis as an example. The characteristics of 78 hospitalized children with steroid-sensitive nephrotic syndrome were studied. The median LOS of the patients was 47 days. Most of the patients' mothers (92.3%) stayed in hospital during admission to care for their children. Using multivariate Cox's regression analysis with time-dependent covariates, non-working mother and smaller number of siblings were found to be factors significantly associated with prolonged LOS. These factors are presumed to allow the mothers to care for their hospitalized children for a long period. No medical or clinical factors played a significant role in the prolonged LOS. These results suggest that sociocultural factors have a greater influence on the LOS in Japan than do medical factors such as disease course, coexisting illness, complications, and treatment response. © 2005 Elsevier Ireland Ltd. All rights reserved.