['The aim of this study was to compare the characteristics of patients suffering from complex regional pain syndrome type 1 ([CRPS], also known as reflex sympathetic dystrophy or algodystrophy) of the knee with those of a matched group of patients experiencing post-traumatic knee pain. The comparisons looked at biopsychosocial complexity (simple versus complex), psychiatric comorbidity and pain intensity.', "Cross-sectional, single-centre, case-control study using the INTERMED scale, psychiatric diagnostic tools (ICD-10) and visual analogue pain scale. Contingency table and Chi2 tests. One-way analysis of variance for continuous dependent variables. Difference between groups: Tukey's variance test, after the event. For nominal dependent variables, multivariate logistic analysis.", 'Biopsychosocial complexity did not differ between the two groups (p = 0.7). The "complex" patients displayed significantly more psychiatric comorbidity (odds ratio 2.94, 95% confidence interval 1.1-7.8, p < 0.01), independently of whether or not CRPS was present. The pain perceived varied with biopsychosocial complexity only in the control group. The "complex" control patients reported more pain than the "simple" control patients (p < 0.05). The perceived intensity of pain was not different between the "simple" and "complex" patients with CRPS.'] Is biopsychosocial complexity correlated with psychiatric comorbidity but not with perceived pain in complex regional pain syndrome type 1 ( algodystrophy ) of the knee?