Evidence on variation in the frequency of health problems between socio-economic groups in the Dutch population has accumulated rapidly in recent years. This paper presents a review of these recent data. It is clear now that a lower socio-economic status is associated with a higher frequency of a wide range of health problems. This negative association has consistently been found for the following health indicators: birth weight; adult body height; prevalence of health complaints; prevalence of many chronic conditions; prevalence of disability; incidence of long-term work incapacity; perceived general health; adult mortality. Inconsistent findings were reported for: children's body height; prevalence of some chronic conditions; incidence of sickness absence (short-term work incapacity); perinatal mortality. The magnitude of the differences varies from study to study, and possibly from health problem to health problem. In studies categorizing the study population in 3-6 hierarchically ordered socio-economic groups on the basis of either education or occupational status, the Relative Risks (of the lowest versus the highest socio-economic group) mostly lie between 1 and 2. Exceptions are prevalence of disability and incidence of long-term work incapacity where Relative Risks between 2 and 4 have been found. A direct comparison with data from other countries is problematic, but at first sight the differences as observed in the Netherlands seem to be of the same order of magnitude as those observed in other industrialized countries. Although most Relative Risks imply 'weak associations' from a technical-epidemiological point of view, the Population Attributable Risks are substantial (generally between 0.25 and 0.40), underlining the public health impact of socio-economic health differences. Information on trends in health inequalities over time is limited to children's body height and adult mortality. For children's body height a substantial decrease of inequalities was found between 1964-1966 and 1980. For adult mortality, on the other hand, there is (indirect) evidence of a widening of the mortality gap between the 1950s and the 1980s. The evidence on specific factors which are involved in the 'causal chain' between socio-economic status and health problems is rather limited at the moment. A negative association with socio-economic status has been reported for the following risk factors: smoking; obesity; a number of unfavourable material living conditions; a number of unfavourable physical working conditions; psychosocial stress; lack of social support; less adequate supply/use of health care.(ABSTRACT TRUNCATED AT 400 WORDS)