Recent evidence suggests the rise in urinary albumin excretion preceding diabetic nephropathy may represent a continuum. We therefore studied factors relating to albumin excretion rate in children with insulin-dependent diabetes. Normal overnight albumin excretion rate was determined in 690 healthy schoolchildren. The 95th centile was 7.2 micrograms min-1. Patients included 169 children with IDDM aged 12.4 +/- 3.1 years who performed 4.8 +/- 0.4 overnight collections during 15 +/- 0.5 months and were analysed cross sectionally. They were stratified accordingly to mean albumin excretion rate: normal < 7.2 micrograms min-1, borderline 7.2-20 micrograms min-1, microalbuminuria 20-200 micrograms min-1; 96/169 patients performed 6.4 +/- 0.2 overnight collections during 24 months follow-up and were analysed longitudinally. Cigarette smoking was determined by history and urine cotinine levels. Smoking correlated with albumin excretion rate, independent of age and other variables, in cross-sectional and longitudinal analysis (p < 0.003). Smoking was more prevalent in the borderline albuminuria and microalbuminuria groups (p < 0.004, p < 0.001). Mean HbA1c during follow-up and mean HbA1c since diagnosis were significantly higher in the microalbuminuric group, compared with the normal patient group. HbA1c since diagnosis, mean blood pressure, lipoprotein(a), and apolipoprotein B did not correlate with albumin excretion rate, after controlling for other variables. Our findings highlight the continuing need for strategies to prevent smoking in this age group.