Desmitis of the accessory ligament of the deep digital flexor tendon (ALDDFT) was diagnosed in 27 horses between September 1986 and December 1990. The first observed clinical sign in four horses was localised swelling in the proximal metacarpus. Twenty horses became lame suddenly during a work period and most developed swelling within 24 h of exercise. The injury was confirmed by ultrasonographic examination. Ten of 13 horses with uncomplicated desmitis of the ALDDFT resumed full work, within three to nine months of the onset of clinical signs, without recurrence of clinical signs in the period of follow-up (nine months to four years). Three horses presented with concurrent acute desmitis of the ALDDFT and superficial digital flexor (SDF) tendonitis and two horses had a history of SDF tendonitis; none of these resumed full work.