Objective: Postpartum psychiatric disorders with long‐lasting adverse sequelae are common during the childbearing years. These disorders can be severe and resistant to conventional psychiatric treatment methods. We present two consecutive cases with puerperal psychosis who were refractory to conventional treatment methods but responded successfully to oestrogen therapy. Method: Serum oestradiol concentration was measured by radioimmunoassay and the documented oestradiol deficiency replaced with physiological oestradiol sublingually. The treatment effect was evaluated by the Brief Psychiatric Rating Scale. Results: In both cases the baseline oestradiol concentration was low (28 and 69 pmol/L). During the treatment with oestradiol, there was a concomitant elevation of the concentration of serum oestradiol, which coincided with the decline in psychotic symptoms. Conclusion: The observation of low serum oestradiol together with psychotic symptoms and successful treatment with oestradiol suggests that oestradiol may have a causal relevance to puerperal psychosis and significance in the treatment of this condition.