Eighty-two mother–infant dyads, comprising women with psychiatric disorder and individually matched controls, were followed up over the children's 1st year of life. The mothers with mental illness consisted of two subgroups: first, 25 severely mentally ill mothers who had been admitted to a psychiatric unit with their infants; and second, 16 mothers from a community sample meeting research diagnostic criteria for unipolar, nonpsychotic depression. With the exception of six dyads in the in-patient group, observations were made of the mother–infant interaction and the quality of the infant–mother attachment relationship at 12 months. The nature and course of the mothers' illness was also documented. Although few residual symptoms of maternal mental illness were detected at 1 year postpartum, interactional disturbances were evident among the case group dyads. A strong association was revealed between infant–mother attachment quality and maternal diagnosis: a manic episode of illness in the postpartum period was related to security in the attachment relationship, and psychotic or nonpsychotic depression was related to insecurity. Concurrent patterns of mother–infant interaction provided support for this finding.