Homocystinuria (HCU) due to cystathionine β-synthase deficiency (Mudd et al 1964) was independently described by Gerritsen and colleagues (USA) and Carson and colleagues (Northern Ireland) in 1962. The worldwide frequency of HCU has been reported as 1 in 344 000, while that in Ireland is much higher at 1 in 65 000, based on newborn screening and cases detected clinically. The national newborn screening programme for HCU in Ireland was started in 1971 using the bacterial inhibition assay. A total of 1.58 million newborn infants have been screened over a 25-year period up to 1996. Twenty-five HCU cases were diagnosed, 21 of whom were identified on screening. The remaining four HCU cases were missed and presented clinically; three of these were breast-fed and one was pyridoxine responsive. Twenty-four HCU cases were pyridoxine nonresponsive. Once the status of pyridoxine responsiveness was identified, all pyridoxine nonresponsive cases, but one, were started on a low methionine, cystine-enhanced diet supplemented with pyridoxine, vitamin B12 and folate. Dietary treatment commenced within 6 weeks of birth (range 8–42 days) for those cases detected by screening, while for the late-detected cases treatment was started upon presentation and diagnosis. Biochemical control was monitored measuring deproteinized plasma methionine, free homocystine and cystine at least once a month. Review of the clinical outcome of the 25 HCU cases with 365.7 patient-years of treatment revealed no HCU-related complications in 18 screened, dietary-treated cases. Fifteen of these had lifetime medians of free homocystine ≤11 μmol/L (range 4–11). The remaining three cases with higher lifetime medians of free homocystine (18, 18 and 48 μmol/L) have developed increasing myopia recently. Among the three screened non-dietary-compliant cases, two have ectopia lentis, one has osteoporosis and two have mental handicap. Of the four cases missed on screening, three presented with ectopia lentis after the age of 2 years. There were no thromboembolic events in any of the 25 HCU cases. The lifetime medians for methionine ranged from 47 to 134 μmol/L. The Irish HCU clinical outcome data suggest that newborn screening, early commencement of dietary treatment and a lifetime median of free homocystine of ≤11 μmol/L had significantly reduced the probability of developing complications when it was compared to the untreated HCU data (Mudd et al 1985).