Diagnostic and demographic data were collected from all 2,298 psychiatric hospital admissions for affective illnesses from private households in the City of Plymouth for the six-year period 1970–1975 inclusive. Intercorrelations of diagnostic subtypes were performed, together with a multiple regression analysis against spatial and ecological data from the 1971 Census. Rate differences were related to the geographic structure of Plymouth. With psychotic illnesses, ecological correlations were low for male and female first admissions and for male readmissions. However, important correlations relating to socio-economic status, housing tenure and structure, population instability, and other sociodemographic features emerged in varying degrees of specificity for reactive and neurotic illness in males, and for all readmissions in females, largely irrespective of diagnostic subtype. Explanations for the processes underlying these patterns are offered in terms of population structure, particularly the differing vulnerability of age and marital status groups, the referral and diagnostic process, social and physical stresses in the lower socioeconomic groups, and urban drift.