The role of paradoxical puborectalis contraction in the aetiology of constipation and how to best diagnose this condition is controversial. The aims of this study were to investigate whether absolute or relative paradoxical electrical activity during electromyography (EMG) are related to rectal emptying and to compare EMG, defecography and digital examination in the diagnosis of paradoxical puborectalis contraction. Included in the study were 171 consecutive patients with idiopathic constipation; 136 of these cases were also classified as paradoxical or unclear or not paradoxical at digital examination. Absolute amplitudes and a strain/squeeze index were used to grade the EMG activity in the puborectalis and external sphincter muscle. Rectal evacuation was analysed by defecography with image analysis of rectal area. The results showed that 142 patients had paradoxical EMG activity during straining. There was a correlation between rectal evacuation and amplitudes (r = -0.20 to -0.03, P < 0.01) and between evacuation and index (r = -0.34 to -0.39, P < 0.0001). Forty-two patients with an index of > 50 had impaired rectal evacuation compared with those with an index < or = 50 (P < 0.0001). Thirty-three of 34 cases (n = 136) with an index of > 50 also were paradoxical at defecography whereas 19 were diagnosed digitally. In conclusion, paradoxical puborectalis contraction is associated with impaired rectal evacuation. The activity seems to be best reflected by a strain/squeeze index. The best correlation in diagnostic methods was between EMG and defecography.