Over the past 20 years, rates of sickness absence have increased and psychiatric disorders have become an important cause of sickness absence. The socio-demographic associations for psychiatric sickness absence are reported from the Whitehall II study, a longitudinal survey of 10,308 London-based male and female civil servants between 35-55 years. Short spells (< or = 7 days), long spells (> 7 days) and very long spells (> 21 days) of sickness absence were examined in 5620 civil servants for whom reason for absence was available in civil service records. Civil service coding of reasons for absence was validated against report of general practitioners' diagnoses. Psychiatric disorder, largely neurosis and neurosis ill-defined, was the third most common cause of long spells of sickness absence in women and the fourth most common in men. For both men and women it was the second most common cause of very long spells of absence. Psychiatric sickness absence for short, long and very long spells was more frequent in lower employment grades than higher employment grades in keeping with the pattern for other illnesses. This partly explains the higher rate of sickness absence in women than men. Widowed and single men, and divorced women had high rates of psychiatric sickness absence. Comparing reason codes based on sickness certificates with general practitioners reports, there appeared to be evidence of under-reporting of psychosis on certificates.(ABSTRACT TRUNCATED AT 250 WORDS)