Phosphatidylglycerol (PG) and lecithin/sphingomyelin ( L S) ratios were stimultaneously determined in 1770 samples of amniotic fluid. PG was defined as "present" when it appeared in at least 2% of the total phospholipids. Comparison of the appearance of PG in samples of amniotic fluid obtained from diabetic and nondiabetic pregnant patients according to L S ratios showed that samples from diabetic patients were less likely to have PG present than those from nondiabetic patients, particularly at L S ratios between 3.1 and 4.0. Because of that, further studies compared the appearance of PG according to gestation in 186 diabetic patients with that in a group of 238 control patients that was composed only of those who underwent repeat cesarean section. Rh-sensitized patients were not included in the control group, since they had delayed appearance of PG. The results showed that the appearance of PG was delayed in patients with Classes B, C, and D-F-R diabetes. In patients with Class A diabetes, the appearance of PG was not significantly different from that in control patients over the entire gestational period, although the appearance of PG may be delayed during early gestation. In contrast, there were no significant differences in L S ratios in various classes of diabetic pregnant patients and control patients at different gestations.