Objective : To evaluate the effectiveness of a health visitor led intervention for failure to thrive in children under 2 years old. Design : Controlled trial, randomised by primary care practice. Setting : Newcastle upon Tyne health district. Intervention : Structured health visitor management, with dietetic, paediatric, and social work input as required. Subjects : 229 children (120 in intervention practices and 109 in control practices) were identified as failing to thrive by population screening during the first 2 years of life. Follow up was by home visit of a research nurse and review of the childrens' records at age 3 years. Main outcome measures : Follow up weight and height and number of routinely collected weights. Results : 95 of the 97 families offered intervention completed at least the initial assessment. At follow up, 187 (82%) records were reviewed, and these suggested that 15 (16%) controls were lost to follow up immediately after the screening weight was taken compared with only one child in the intervention group. In the 134 (58%) families who consented to home visits, children in the intervention group were significantly heavier and taller and were reported to have better appetites than childen in the control group, although both groups were equally satisfied by the services they had received. When the children were last weighed, 91 (76%) in the intervention group had recovered from their failure to thrive compared with 60 (55%) in the control group (P<0.001). Conclusion : In failure to thrive, health visitor intervention, with limited specialist support, can significantly improve growth compared with conventional management.