Methods: Cross-sectional study of 11 patients with tyrosinaemia III (4 males, 7 females), age range 2-17 years (median 8) using age-appropriate cognitive tests. A questionnaire was completed by school relating to behaviour and special educational needs. Age at which dietary treatment was commenced and the pre-diet and post-diet plasma tyrosine levels until age 5 years were recorded. Results: Pre-diet plasma tyrosine concentrations were between 500 and 1500 μmol/l. Median age at commencement of dietary treatment was 12 weeks (range 5-112 weeks). Post-diet tyrosine concentrations ranged from 60-300 μmol/l. 75% of IQ scores were borderline (70-80) or extremely low (<70) with no significant differentiation between verbal and nonverbal scores. 2 patients had special educational needs and 2 had concentration difficulties. There was a weak negative correlation between the age at which patients started dietary treatment and full scale IQ (r=−0.514; p=0.192 Objectives: to examine the longterm outcome of treated homocystinuria patients (HCU, CbS deficiency), and to examine ways to quantify patient exposure to free homocystine and total homocysteine. Methods: A retrospective case note review of 46 HCU patients (92% pyridoxine nonresponsive), born between 1971-2009. Patients were divided into 4 groups: 23 New Born Screening (NBS) good/moderate control (Group I); 7 NBS poor control (Group II); 10 not detected on NBS (Group III); 6 born pre-NBS (Group IV). Median age of 28.23 years (0.4 -54.6 ) included 950 treatment years. Free homocystine measured between 1971-2009, and total homocysteine from 1997. Alternative statistical models were used to determine control: area under HCy curve; and expected homocystine level (free homocystine exposure/ years of exposure); level of exposure to free homocystine to time of clinical event.