Oligomeganephronia is a rare congenital renal hypoplasia that is characterized histologically by a reduction in the number of reniculi with compensatory hypertrophy of the glomeruli and proximal renal tubules. Oligomeganephronia has generally been regarded as a congenital but not genetic disease. Kusuyama et al. first suggested that oligomeganephronia might be associated with a chromosomal anomaly, namely 4p monosomy syndrome. Their assumption originated from the fact that external anomalies of their cases of oligomeganephronia are very much like those described in 4p monosomy syndrome. We have experienced two autopsy cases of oligomeganephronia associated with multiple congenital anomalies that are seen in 4p deletion syndrome. Chromosome studies performed in both cases revealed 4p deletion and 4p ring, respectively. There were remarkable similarities between these cases. We suggest that there are two types of oligomeganephronia; one is a solitary sporadic type with no associated anomaly, and the other is a syndromic type that is a part of a complex anomaly of 4p deletion syndrome and possibly other related chromosomal deletion syndromes.