Yeasts may gain entry into the blood via routes such as intubation, intravenous catheterization or by direct persorption from the gut. The latter route becomes important when the numbers of commensal yeasts in the gut exceeds a threshold which may vary between animal species. In a prospective study utilizing serial, twice weekly quantitative stool cultures during the first 6 weeks of life of 40 very low birth weight infants, we found a threshold of 8 x 10(6) Candida colony-forming units/gram of stool. Beyond this threshold 50% of the infants developed gastrointestinal symptoms and 28.5% developed systemic sepsis within 1 to 3 weeks of heavy colonization. The gastrointestinal colonization rate was 62.5% (25/40) with 66% having Candida colony-forming units greater than 8 x 10(6)/g stool.