Urinary galactose and galactitol excretion in controls is age-dependent with the highest concentrations at a younger age. Untreated patients with classical galactosemia excreted highly elevated amounts of galactitol (8000-69,000 mmol/mol creatinine; controls 3-81) which did not correlate with galactose excretion. After treatment, galactose excretion returned to normal in all patients whereas galactitol excretion (45-900 mmol/mol creatinine) remained above the age-matched control range. The excretion of galactitol (96-170 mmol/mol creatinine) in untreated compound heterozygotes was much lower although still above the age-matched control levels, and it returned to normal after treatment. In untreated classical galactosemia patients the galactitol in plasma (120-500 mumol/l) was markedly elevated (controls 0.08-0.86 mumol/l); under treatment, the galactitol concentrations (4.7-20 mumol/l) remained above the control range in all. There was no correlation with age nor with galactose-1-phosphate and UDP-galactose levels. Two untreated compound heterozygotes had elevated plasma galactitol (6.0 and 63 mumol/l) which, when treated, returned to normal.