Asthma admission records in London (2001Á2006) were used. Negative binomial regression was used to model the effect of demographic (sex, age & ethnic group), diagnostic (primary & secondary diagnosis, method of admission) and temporal (day of the week, meteorological season & year of admission) factors on the LOS, accounting for the random effects of each patient's attendance, as model 'I' and again for area of residence, model 'A'. Akaike information criterion (AIC) was used to compare the two models.