The aim of the study was to investigate the sleep patterns and the prevalence and types of sleep problems among urban Chinese children under 5 years of age. Data were gathered on 14,883 children selected by stratified random sampling from 12 cities, using the Children's Sleep Status Questionnaire (CSSQ). The parents were asked about their children's average sleep time in a 24-hour period and common sleep problems noted during the past month. Of this sample, 20.3% of the children were reported as suffering from at least one of the 14 sleep disturbances studied. The five most common sleep problems were difficulty falling asleep (found in 7.5% of pre-school children), nocturnal awakening (4.9%), bruxism (4.7%), snoring (4.4%) and mouth breathing (3.0%). Excluding the cases with sleep disorders, the average sleep time over a 24-h period progressively shortened with age, from 15.8 hours at 1 month to 13.1 hours at 6 months, 12.6 hours at 12 months, 12.0 hours at 2 years, and 11.3 hours at 5 years. The number of naps and nocturnal awakenings, respectively, decreased from 4 and 2 per day at 1 month of age to 1 and 0 per day at 24 and 18 months; most children continued to nap up to the age of 5 years. Our findings suggest that sleep problems are common in Chinese children aged 0-5 years. We also found that, with the development of society, certain changes in sleep time of the children have occurred, there being about 0.5-1.0hour less sleep than is commonly reported in medical textbooks.