Koro is an unusual psychogenic syndrome reported, until recently, predominantly in men of the Chinese race who live in southern China and south-east Asia. Issues concerning its phenomenology, diagnosis and nosology are still controversial. This paper describes an epidemic of koro in north-east India. A psychiatric analysis of thirty-one cases is presented. Probable reasons for the rapid spread of the illness are discussed. The majority of the individuals affected were from the lower socio-economic strata, were poorly educated and in the age group of 20-40 years. Many women were affected. There was no evidence of significant premorbid or sexual psychopathology in most cases. Some patients had a number of episodes but with only minor residual symptoms. The author compares these findings with earlier reports and discusses the implications for its nosology and psychodynamics.