A girl with the syndrome of thrombocytopenia with absent radius had severe diarrhoea and dehydration relieved by withdrawal of cows' milk and aggravated by its reintroduction on three occasions. Deterioration in gastrointestinal symptoms was associated with haematological relapse with thrombocytopenia, leucocytosis, anaemia, and eosinophilia. There appeared to be a correlation between milk exposure and the haematological and gastrointestinal disturbances. Supporting evidence from published reports for such a correlation is reviewed. Cows' milk protein intolerance may be a factor in precipitating haematological relapse in susceptible infants with radius aplasia. Early withdrawal of cow's milk protein should be tried in thrombocytopenia with absent radius, especially in cases with prominent gastrointestinal upset.