Methods. Information on 256 HIV-infected mothers reported to the CPARG surveillance and supplemented using Motherisk and HIV Program and chart reviews was used. Logistic regression modeling and tests of association were used to analyze the data.Results. 33% were diagnosed before conception and 27% after delivery; 96% of HIV transmissions occurred among the latter. Proportion diagnosed after delivery decreased over time. Year of delivery was independent correlate of diagnosis after delivery; age and country of origin for diagnosis before conception. Mothers diagnosed before conception were older, mostly Canadian and fertility rate was lower than general population's.Objective. To examine patterns of HIV diagnosis and correlates of late HIV diagnosis among HIV-infected mothers giving birth in Ontario from 1994 to 2002.Conclusion. HIV prenatal testing since 1994 has increased diagnosis during pregnancy but the rate remains suboptimal. No specific subgroups of women, notably women from HIV-endemic countries, are being selectively missed.