Recent reports have suggested that fascicular constriction located proximal to the elbow may be the primary aetiology of spontaneous anterior interosseous nerve palsy (sAINP), and recommended interfascicular neurolysis ranging from the proximal forearm to distal upper arm (wide incision surgery: WIS) for its treatment. On the basis of these reports, it was hypothesised that, if the aetiology of sAINP was fascicular constriction, neurolysis limited proximal to the elbow (minimal incision surgery: MIS) should have similar results to those of WIS. Twenty-five surgically treated sAINP cases were retrospectively evaluated. The mean age of onset was 42.8 years, the mean preoperative period was 5.3 months, and mean follow-up period after operation was 31 months. Eleven patients underwent MIS, while 14 underwent WIS according to the patient's intention (intention-to-treat). Twenty-one patients had fascicular constriction, and no entrapment neuropathy was seen in this series. At the latest follow-up, 82% of the patients had British Medical Research Council grade of ≥4 in flexor-pollicis-longus, while 80% had grade of ≥4 in flexor-digitorum-profundus-of-the-index-finger. There were no significant differences between the results of either group. These result suggested that the aetiology of sAINP may not be external compression, but fascicular constriction, and MIS may be its favourable surgical treatment.