The objective of this study is to analyze the effects of residential substance abuse treatment on pregnancy outcome among gravidas in a gender-specific program. All clients (cases) who entered a residential substance abuse program for pregnant and postpartum women were eligible for inclusion in the study (n=95). Only those who were in treatment at the time of delivery were included in the present analysis (n=57). Two comparison groups were used: (1) substance abusers who received no treatment during pregnancy (positive control group) and (2) pregnant women who were not substance abusers (negative control group). Cases were matched to controls on ethnicity (negative and positive controls) and drug of choice (positive controls only). Medical records were reviewed and abstracted for cases and controls. The primary drug of choice was cocaine for 56% of clients in the study, heroin 15.8%, and alcohol 10.8%. Average length of time in treatment before delivery was 11.7 weeks. The frequency of pregnancy complications allowing treatment and position controls was significantly higher than the negative control group (p<0.0001). The frequency of perinatal infant complications was increased among treatment group infants (p<0.0001). Two infants in the treatment group were positive for a substance of abuse at birth. In the treatment versus positive control group, mean birth weight (BW) was 3227 versus 2800 g (p<0.01), estimated gestational age (EGA) was 38.9 versus 39 weeks, average head circumference (FOC) was 33.8 versus 32.5 cm (p<0.05), and mean birth length (BLT) was 48.7 cm versus 46.9 (p<0.05). No significant differences were found between treatment and negative control groups. Maternal syphilis was increased in frequency in the positive control group compared with the negative control group (p<0.07). Thirty-percent of mothers had sexually transmitted diseases (STDs) for which infants were at risk and treated prophylactically; no infant in the treatment group contracted a vertically transmitted STD. For every 10 weeks in treatment, BW was increased 340 g, EGA 1 week, FOC 0.8 cm, and BLT 1.8 cm. Thus, substance abuse treatment for pregnant women in the program increased fetal growth, which significantly decreased the risk for poor neonatal outcomes. Importantly, maternal and infant perinatal complications in the treatment group were increased in frequency compared with the two control groups. This may possibly have occurred because healthcare providers were not blinded to maternal treatment status.