A12 month old female infant born at full term by spontaneous vaginal delivery presented at Sultan Qaboos University Hospital, Oman. Her birth weight was 3.23kg, length 50cm and head circumference 34.5cm, all appropriate for her gestational age. The parents were first cousins and had lost three male children due to repeated severe infections. This infant developed chronic diarrhoea, recurrent chest and skin infections from the age of six months, and subsequently had failed to thrive with a weight, height and head circumference < 3rd centile for age. The physical examination revealed subtle facial dysmorphic features, partial bilateral ptosis, generalized hypotonia, mild hepatomegaly and developmental delay. Pearson's marrow-pancreas syndrome was first described by Pearson et al. in 1979. This is a rare multi-system disorder with a poor prognosis. The defining features are marrow failure, anaemia, neutropenia, thrombocytopenia and characteristic vacuolation of haematopoietic precursors. An additonal defining feature of Pearson's syndrome is dysfunction of the exocrine pancreas due to fibrosis and acinar atrophy resulting in malabsorption and chronic diarrhoea with fatty stool. Another cardinal feature of Pearson's syndrome is persistent or intermittent lactic acidaemia, which is caused by a defect in oxidative phosphorylation. Variable hepatic, renal, and endocrine failure may also occur.2,3,4 The genetic defect in Pearson's marrow pancreas syndrome has been found to be a deletion in the mitochondrial DNA. The most common deletion reported is a 4977 base pair deletion identified in > 80% of affected children. In the literature, there is a clear evidence of clinical phenotype heterogeneity which has been attributed to genetic variability in the proportion of mt DNA deletions.