This study compared psychological distress between immigrants from high- and low-income countries living in Oslo, and investigated whether pre- or post-migration factors could explain any such differences in distress levels. A cross-sectional survey with self-administered questionnaires was conducted between 2000 and 2001 among 812 immigrants from high-income countries and 1434 immigrants from low-income countries living in Oslo. The Hopkins Symptom Checklist (HSCL)-10 was used to measure psychological distress. The prevalence rates of psychological distress among immigrants from high- and low-income countries were 10.3% and 24.3%, respectively (P=0.001). Unadjusted odds ratio (OR) of distress among the latter group was 2.38 with 95% confidence interval (CI) 1.73-3.29. The first adjustment (socio-demographic variables) hardly attenuated the difference reported (adjusted OR =2.25, 95% CI 1.58-3.21), the second adjustment (socio-demographic and pre-migration variables) reduced the difference (adjusted OR =1.86, 95% CI 1.28-2.69) while the last adjustment (socio-demographic and pre- and post-migration variables) attenuated the difference below the level of significance (adjusted OR =1.33, 95% CI 0.88-2.01). This means that both pre-migration and post-migration factors were associated with the higher level of distress among immigrants from low-income countries, the post-migration factors in the host country probably being the most important.