The report on the influence of seasonal factors on thymic size in early life describes a pattern of ultrasonographically measured thymic growth in Gambian infants including the finding of a smaller thymus in the hungry season. These factors raise a number of important questions: Is the size of the thymus relevant to its function and could measurement of the thymus be a useful immunological diagnostic tool in the investigation of thymic function in humans with a depressed immune system? Conclusion: Studies using the size of the thymus as an immunological diagnostic tool should be encouraged.