In this study we evaluated the acceptability of using the first morning urine albumin concentration (FMAC) and the first morning urine albumin/creatinine (FMA/C) ratio as an indirect estimation of timed albumin excretion in order to screen for microalbuminuria in a large diabetic population. Urinary albumin excretion rate (AER) was determined in samples from 4-h urine collection in 99 type 1 diabetic patients aged 30 +/- 10 years with a mean duration of diabetes of 15 +/- 8 years. The results of timed albumin excretion were successively compared with single-void first morning samples. On the basis of AER, 46 patients were normoalbuminuric (AER less than 20 micrograms/min), 28 microalbuminuric (AER 20-200 micrograms/min), and 25 proteinuric (AER greater than 200 micrograms/min). The relationship of 4-h AER to FMAC and FMA/C ratio was highly significant (r = 0.96 and r = 0.98 respectively). High sensitivity and specificity were found when cut-offs of 20 micrograms/ml and 2.5 mg/mmol were selected for albumin concentration and albumin/creatinine ratio respectively to discriminate between normal and elevated albuminuria. It is concluded that the measurements of albumin concentration and albumin/creatinine ratio in first morning urine samples are highly representative of 4-h timed albumin excretion. Because of their sensitivity, specificity and simplicity to perform, the tests proposed might be used in routine diabetic care and as a screening test for microalbuminuria in type 1 (insulin-dependent) diabetic patients. The not negligible day-to-day variability in albumin excretion confirms the need of several measurements to establish the presence of abnormal levels of albuminuria above all in patients with borderline values and/or clinically unstable metabolic control.