An abnormality in galactosylation of complex carbohydrates may be important in the pathogenesis of the long-term complications of classic (galactose-1-phosphate uridyltransferase-deficient) galactosemia. The ability of nine galactosemic fibroblast preparations to be galactosylated with a purified galactosyltransferase was measured as an indicator of vacant sites where galactose would normally reside. The amount of galactose transferred to cell protein from galactosemic patients was significantly higher than that transferred to a group of seven controls (p less than 0.005). Galactosyltransferase activity of the galactosemic cell preparation toward N-acetylglucosamine was also significantly higher than normal (p less than 0.01), and there was a linear relationship between these two parameters in galactosemic but not normal cells. These findings suggest that there is defective galactosylation of galactosemic cell complex carbohydrates and that such cells increase their galactosyltransferase levels in an attempt to compensate for the defect. Defective galactosylation may be implicated as an etiologic factor in complications observed in galactosemic patients even when treated with galactose-restricted diets.