Methods: HIV-1 seropositive pregnant women and seronegative controls were enrolled at a maternity hospital in Nairobi. Women and their children were followed from birth, and data on HIV-1 transmission, breastfeading, clinical illness, and growth were collected. Specimens for HIV-1 serology and/or polymerase chain reaction were obtained at birth, 2, 6, and 14 weeks, 6, 9, 12, and 18 months, and every 6 months thereafter. Children were classi®ed as HIV-1 uninfected, perinatally, or postnatally

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... ected. Potentially breastfeeding transmission related risk factors were compared between postnatally infected and uninfected children. Results: Among children born to seropositive or seroconverting mothers, 317 were uninfected, 51 infected perinatally and 42 infected postnatally. Identi®ed risk factors for postnatal transmission were maternal nipple lesions (OR 2.3, CI 95% 1.1±5.0), mastitis (OR 2.7, CI 95% 1.1±6.7), maternal CD4 cell count , 400 mm 3 (OR 4.4, CI 95% 1.9±9.9), maternal seroconversion while breastfeeding (OR 6.0, CI 95% 1.8±19.8), infant oral thrush at , 6 months of age (OR 2.8, CI 95% 1.3±6.2) and breastfeeding longer than 15 months (OR 2.4, CI 95% 1.2±5.1). All factors, except maternal seroconversion due to its rarity, were independently associated with an increased postnatal transmission risk by multivariate logistic regression analysis. Conclusion: In addition perinatal antiretroviral therapies, public health strategies should address: (i) prevention of maternal nipple lesions, mastitis and infant thrush; (ii) reduction of breastfeeding duration by all HIV-1-infected mothers; (iii) absolute avoidance of breastfeeding by those at high risk, and (iv) prevention of HIV-1 transmission to breastfeeding mothers.