Ascaris lumbricoides, the roundworm, is one of the largest parasites of man and probably infects one in four persons in the world. Despite its prevalence, ascariasis is a largely neglected public health problem that has attracted relatively little scientific inquiry. Frequently, a number of biases contribute to the uncritical conclusion that infection with A. lumbricoides adversely affects the nutritional status of the host. This situation is exacerbated by a number of studies that have confirmed these biases but have employed questionable methods, such as the use of small samples and indistinct categories, the neglect of the double-blind safeguard, the selection of inadequate controls, and the performance of experiments that are not reproducible in a variety of circumstances. It is interesting to note that studies claiming positive correlation between ascariasis and protein energy malnutrition have not found a significant difference in weight between infected and uninfected children before intervention. Furthermore, several recent studies have shown no significant improvement in nutritional status after intervention. Thus, the causal relationship between ascariasis and protein energy malnutrition is not clearly proved, and it is premature to advocate mass treatment of children in ascariasis-endemic areas as a method to enhance their growth and development.